[url_launcher] Improved documentation of the headers parameter.

[Sempakonka]

Description

Added a clear documentation of the fact that the headers parameter at the launch method is not fully supported when using an external browser.

Related Issues

Issue 61332.

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[Sempakonka]

@googlebot I signed it!

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[amirh]

I think that a reader of this comment is looking to understand whether and under what circumstances he can use this. Maybe something like:

When not using a WebView, the header information is passed to the browser, some Android browsers do not support the [Browser.EXTRA_HEADERS](https://developer.android.com/reference/android/provider/Browser#EXTRA_HEADERS) intent extra and will lose the header information.

[amirh]

LGTM