docs: Put window.open docs in its own chapter, regards #1137

docs/api/window-open.md

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+# `window.open` function
+
+When `window.open` is called to create a new window in web page, a new instance
+of `BrowserWindow` will be created for the `url`, and a proxy will be returned
+to `window.open` to let the page to have limited control over it.
+
+The proxy only has some limited standard functionality implemented to be
+compatible with traditional web pages, for full control of the created window
+you should create a `BrowserWindow` directly.
+
+## window.open(url, [frameName[, features]])
+
+* `url` String
+* `frameName` String
+* `features` String
+
+Creates a new window and returns an instance of `BrowserWindowProxy` class.
+
+## window.opener.postMessage(message, targetOrigin)
+
+* `message` String
+* `targetOrigin` String
+
+Sends a message to the parent window with the specified origin or `*` for no
+origin preference.
+
+## Class: BrowserWindowProxy
+
+### BrowserWindowProxy.blur()
+
+Removes focus from the child window.
+
+### BrowserWindowProxy.close()
+
+Forcefully closes the child window without calling its unload event.
+
+### BrowserWindowProxy.closed
+
+Set to true after the child window gets closed.
+
+### BrowserWindowProxy.eval(code)
+
+* `code` String
+
+Evaluates the code in the child window.
+
+### BrowserWindowProxy.focus()
+
+Focuses the child window (brings the window to front).
+
+### BrowserWindowProxy.postMessage(message, targetOrigin)
+
+* `message` String
+* `targetOrigin` String
+
+Sends a message to the child window with the specified origin or `*` for no
+origin preference.
+
+In addition to these methods, the child window implements `window.opener` object
+with no properties and a single method: