clarify docs for unsupportedMembers.

some problems with the previous wording:
- "each such member" is ambiguous since the previous paragraph talks about
  2 or perhaps 3 kinds of members:
  - 1 where the return type is non-nullable and unknown
  - 1 where it's that plus it's specified in unsupportedMembers
  - presumably 1 more where it's that plus not in unsupportedMembers
- typo: "with throws"
- the hypothesis "if unsupportedMembers contains the name of each such member"
  makes one wonder what unsupportedMembers does in general.
  is this asserting that client code must put such members into
  unsupportedMembers?
  is unsupportedMembers allowed to contain more than that?
- "generated with an override" should probably apply to the member,
  not the class

PiperOrigin-RevId: 439901387

Author: srawlins

lib/annotations.dart

@@ -88,11 +88,12 @@ class MockSpec<T> {
   /// override member, unless the member is specified in [unsupportedMembers],
   /// or a fallback implementation is given in [fallbackGenerators].
   ///
-  /// If [unsupportedMembers] contains the name of each such member, the mock
-  /// class is generated with an override, with throws an exception, for each
-  /// member with a non-nullable unknown return type. Such an override cannot be
-  /// used with the mockito stubbing and verification APIs, but makes the mock
-  /// class a valid implementation of the class-to-mock.
+  /// For each member M in [unsupportedMembers], the mock class will have an
+  /// override that throws, which may be useful if the return type T of M is
+  /// non-nullable and it's inconvenient to define a fallback generator for M,
+  /// e.g. if T is an unknown type variable. Such an override cannot be used
+  /// with the mockito stubbing and verification APIs, but makes the mock class
+  /// a valid implementation of the class-to-mock.
   ///
   /// Each entry in [fallbackGenerators] specifies a mapping from a method name
   /// to a function, with the same signature as the method. This function is