docs: update GN developer docs (#14018)

docs/development/build-instructions-gn.md

@@ -3,8 +3,7 @@
 Follow the guidelines below for building Electron with the experimental GN
 build.
 
-> **NOTE**: The GN build system is in _experimental_ status, and currently only
+> **NOTE**: The GN build system is in _experimental_ status.
-> works on macOS, Linux and Windows.
 
 ## Prerequisites
 
@@ -19,13 +18,12 @@ Check the build prerequisites for your platform before proceeding
 You'll need to install [`depot_tools`][depot-tools], the toolset
 used for fetching Chromium and its dependencies.
 
-Also, on windows open:
+Also, on Windows, you'll need to set the environment variable
-
+`DEPOT_TOOLS_WIN_TOOLCHAIN=0`. To do so, open `Control Panel` → `System and
-`Control Panel → System and Security → System → Advanced system settings`
+Security` → `System` → `Advanced system settings` and add a system variable
-
+`DEPOT_TOOLS_WIN_TOOLCHAIN` with value `0`.  This tells `depot_tools` to use
-and add a system variable `DEPOT_TOOLS_WIN_TOOLCHAIN` with value `0`.
+your locally installed version of Visual Studio (by default, `depot_tools` will
-This tells `depot_tools` to use your locally installed
+try to download a Google-internal version that only Googlers have access to).
-version of Visual Studio (by default, `depot_tools` will try to use a google-internal version).
 
 [depot-tools]: http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up
 
@@ -33,15 +31,10 @@ version of Visual Studio (by default, `depot_tools` will try to use a google-int
 
 ```sh
 $ mkdir electron-gn && cd electron-gn
-$ cat > .gclient <<-GCLIENT
+$ gclient config \
-solutions = [
+    --name "src/electron" \
-  {
+    --unmanaged \
-    "url": "https://github.com/electron/electron",
+    https://github.com/electron/electron
-    "managed": False,
-    "name": "src/electron",
-  },
-]
-GCLIENT
 $ gclient sync --with_branch_heads --with_tags
 # This will take a while, go get a coffee.
 ```
@@ -56,18 +49,22 @@ $ gn gen out/Default --args='import("//electron/build/args/debug.gn")'
 
 This will generate a build directory `out/Default` under `src/` with
 debug build configuration. You can replace `Default` with another name,
-but it should be a subdirectory of `out`. Also, to know the list
+but it should be a subdirectory of `out`.
-of available configuration options, run `gn args out/Default --list`.
 Also you shouldn't have to run `gn gen` again—if you want to change the
 build arguments, you can run `gn args out/Default` to bring up an editor.
 
-**For generating Debug/Component build config of Electron:**
+To see the list of available build configuration options, run `gn args
+out/Default --list`.
+
+**For generating Debug (aka "component" or "shared") build config of
+Electron:**
 
 ```sh
 $ gn gen out/Default --args='import("//electron/build/args/debug.gn")'
 ```
 
-**For generating Release/Non-Component build config of Electron:**
+**For generating Release (aka "non-component" or "static") build config of
+Electron:**
 
 ```sh
 $ gn gen out/Default --args='import("//electron/build/args/release.gn")'
@@ -85,7 +82,8 @@ This will build all of what was previously 'libchromiumcontent' (i.e. the
 so it will take a while.
 
 To speed up subsequent builds, you can use [sccache][sccache]. Add the GN arg
-`cc_wrapper="sccache"` by running `gn args out/Default` to bring up an editor.
+`cc_wrapper = "sccache"` by running `gn args out/Default` to bring up an
+editor and adding a line to the end of the file.
 
 [sccache]: https://github.com/mozilla/sccache
 
@@ -93,6 +91,8 @@ The built executable will be under `./out/Default`:
 
 ```sh
 $ ./out/Default/Electron.app/Contents/MacOS/Electron
+# or, on Windows
+$ ./out/Default/electron.exe
 # or, on Linux
 $ ./out/Default/electron
 ```
@@ -100,17 +100,23 @@ $ ./out/Default/electron
 ### Cross-compiling
 
 To compile for a platform that isn't the same as the one you're building on,
-set the `target_cpu` GN argument. For example, to compile a windows x86 target
+set the `target_cpu` and `target_os` GN arguments. For example, to compile an
-from an x64 host, specify `target_cpu = "x86"` in `gn args`.
+x86 target from an x64 host, specify `target_cpu = "x86"` in `gn args`.
 
 ```sh
 $ gn gen out/Default-x86 --args='... target_cpu = "x86"'
 ```
 
 Not all combinations of source and target CPU/OS are supported by Chromium.
-Only cross-compiling Windows 32-bit from Windows 64-bit has been tested in
+Only cross-compiling Windows 32-bit from Windows 64-bit and Linux 32-bit from
-Electron. If you test other combinations and find them to work, please update
+Linux 64-bit have been tested in Electron. If you test other combinations and
-this document :)
+find them to work, please update this document :)
+
+See the GN reference for allowable values of [`target_os`][target_os values]
+and [`target_cpu`][target_cpu values]
+
+[target_os values]: https://gn.googlesource.com/gn/+/master/docs/reference.md#built_in-predefined-variables-target_os_the-desired-operating-system-for-the-build-possible-values
+[target_cpu values]: https://gn.googlesource.com/gn/+/master/docs/reference.md#built_in-predefined-variables-target_cpu_the-desired-cpu-architecture-for-the-build-possible-values
 
 ## Tests
 
@@ -128,7 +134,12 @@ $ (cd electron/spec && npm i --nodedir=../../out/Default/gen/node_headers)
 Then, run Electron with `electron/spec` as the argument:
 
 ```sh
+# on Mac:
 $ ./out/Default/Electron.app/Contents/MacOS/Electron electron/spec
+# on Windows:
+$ ./out/Default/electron.exe electron/spec
+# on Linux:
+$ ./out/Default/electron electron/spec
 ```
 
 If you're debugging something, it can be helpful to pass some extra flags to