Clarify that setting up a FCM Rewrite Proxy is not necessary for use of the UnifiedPush FCM distributor.

[anoadragon453]

Type of change

• Feature
• Bugfix
• Technical
• Other: Documentation

Content

After #6228 was merged, an enterprise chimed in with worry that they'd need to set up an FCM Rewrite Proxy in order to continue using FCM, as is documented here.

After discussing with the UnifiedPush developers however, it appears that Sygnal replaces the need for this in the case of Element Android.

This change adds a bit to the documentation to clarify this.

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

[bmarty]

LGTM, thanks!