docs(compat): state the compatibility API's support status (#3810)

* docs(compat): state the compatibility API's support status

docs/compat.md had no statement of whether @angular/fire/compat is
supported, and the v7 upgrade guide still told readers to stop at the
compatibility API because the modular API was not feature complete,
a sentence written for the 7.0 release.

The new section records the decision for 22: the compatibility API is
supported and kept working, its specs run in the same suite, it
receives bug fixes and no new features, and no removal is planned.
The v7 guide line now points at that section.

* docs(compat): mention the turned-off compat tests

The support section said the compatibility API is kept working, but
CI skips the tests for AngularFirestoreCollection,
AngularFirestoreCollectionGroup, AngularFirestoreDocument and one
Cloud Storage upload. The section now says so and links the issue
that tracks turning them back on.

Author: armando-navarro

docs/compat.md

@@ -9,6 +9,12 @@ AngularFire has a new tree-shakable API, you're looking at the documentation for
 
 [See the v7 upgrade guide for more information on this change.](version-7-upgrade.md).
 
+### Support status
+
+The compatibility API (`@angular/fire/compat/*`) is still supported. It gets bug fixes but no new features, which go into the modular API only. There are no plans to remove it, so you can keep upgrading AngularFire without migrating.
+
+Some of its tests are turned off right now: the ones for `AngularFirestoreCollection`, `AngularFirestoreCollectionGroup` and `AngularFirestoreDocument`, and one Cloud Storage upload test. Until [#3822](https://github.com/angular/angularfire/issues/3822) turns them back on, a bug in those APIs may not be caught before a release, so please report any you find.
+
 ### Monitor usage of your application in production
 
 > `AngularFireAnalytics` provides a convenient method of interacting with Google Analytics in your Angular application. The provided `ScreenTrackingService` and `UserTrackingService` automatically log events when you're using the Angular Router or Firebase Authentication respectively. [Learn more about Google Analytics](https://firebase.google.com/docs/analytics).

docs/version-7-upgrade.md

@@ -18,7 +18,7 @@ AngularFire v7.0 has a compatibility layer that supports the AngularFire v6.0 AP
 
 While not as tree-shakable as the new modular SDK, this allows you to upgrade and take advantage of the benefits of the new SDK ASAP.
 
-**Most developers can stop here for now as the new API isn't feature complete.**
+**You can stop here if the compatibility API covers your needs.** It stays supported and receives bug fixes, but new features land in the modular API only. See [the compatibility guide's support status](compat.md#support-status).
 
 ## **NEW** Modular SDK