Elaborate on what is included

Change-Id: Ie3a2214d37d6bbe645b5e86749dd80448624eda0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/178997
Reviewed-by: Lasse R.H. Nielsen <[email protected]>

Author: mit-mit

docs/process/breaking-changes.md

@@ -35,7 +35,8 @@ abide by the following basic conditions:
 * Must access libraries via the public API (for example, must not reach into
   the internals of a package located in the `/src/` directory).
 
-* Must not rely on an [experiment flag](experimental-flags.md).
+* Must not rely on an [experiment flag](experimental-flags.md), or depend on
+  libraries or APIs that are documented as being experimental.
 
 * Must not circumvent clear restrictions documented in the public API
   documentation (for example, must not mixin a class clearly documented as