Use a shorter doc header for CustomMatcher

Separate the second sentence into a new paragraph so dartdoc does not
pull it in to the header. Rephrase the header to say more than that it
is a base class.

CHANGELOG.md

@@ -1,3 +1,5 @@
+## 0.12.17-wip
+
 ## 0.12.16+1
 
 * Require Dart 3.0

lib/src/custom_matcher.dart

@@ -8,7 +8,9 @@ import 'description.dart';
 import 'interfaces.dart';
 import 'util.dart';
 
-/// A useful utility class for implementing other matchers through inheritance.
+/// A base class for [Matcher] instances that match based on some feature of the
+/// value under test.
+///
 /// Derived classes should call the base constructor with a feature name and
 /// description, and an instance matcher, and should implement the
 /// [featureValueOf] abstract method.

pubspec.yaml

@@ -1,5 +1,5 @@
 name: matcher
-version: 0.12.16+1
+version: 0.12.17
 description: >-
   Support for specifying test expectations via an extensible Matcher class.
   Also includes a number of built-in Matcher implementations for common cases.
@@ -19,3 +19,7 @@ dev_dependencies:
   fake_async: ^1.3.0
   lints: ^3.0.0
   test: ^1.23.0
+
+dependency_overrides:
+  test: 1.25.0
+  test_api: 0.7.3