Mark mixed sandbox as macOS/Windows only

docs/api/app.md

@@ -913,7 +913,7 @@ correctly.
 
 **Note:** This will not affect `process.argv`.
 
-### `app.enableMixedSandbox()` _Experimental_
+### `app.enableMixedSandbox()` _Experimental_ _macOS_ _Windows_
 
 Enables mixed sandbox mode on the app.
 

docs/api/sandbox-option.md

@@ -62,6 +62,7 @@ force `sandbox: true` for all `BrowserWindow` instances.
 
 To enable OS-enforced sandbox on `BrowserWindow` or `webview` process with `sandbox:true` without causing
 entire app to be in sandbox, `--enable-mixed-sandbox` command-line argument must be passed to electron.
+This option is currently only supported on macOS and Windows.
 
 ```js
 let win

spec/api-app-spec.js

@@ -568,6 +568,9 @@ describe('app module', function () {
   })
 
   describe('mixed sandbox option', function () {
+    // FIXME Get these specs running on Linux
+    if (process.platform === 'linux') return
+
     let appProcess = null
     let server = null
     const socketPath = process.platform === 'win32' ? '\\\\.\\pipe\\electron-mixed-sandbox' : '/tmp/electron-mixed-sandbox'