Expand the doc comment for Context (#1902)

This is the main point of interaction for authors of extensions, so docs
cover a wide range of information, including how the descriptions will
be used in failure output.

- Export `ConditionSubject` from `context.dart`.
- Use templates for some parts of the doc that describe universal
  behavior and apply them as macros in the docs for individual methods.
- Make the fields in `Extracted` private. This lets the references in
  docs link to the constructors instead of the fields of the same name.
  The interface intended for external use is the constructors, so things
  are simpler if that is the only exposed API.
- Fix README example to use label callback.

Author: natebosch

pkgs/checks/README.md

@@ -96,7 +96,7 @@ extension CustomChecks on Subject<CustomType> {
   }
 
   Subject<Foo> get someDerivedValue =>
-      context.nest('has someDerivedValue', (actual) {
+      context.nest(() => ['has someDerivedValue'], (actual) {
         if (_cannotReadDerivedValue(actual)) {
           return Extracted.rejection(which: ['cannot read someDerivedValue']);
         }

pkgs/checks/lib/context.dart

@@ -4,14 +4,15 @@
 
 export 'src/checks.dart'
     show
-        Subject,
         CheckFailure,
         Condition,
+        ConditionSubject,
         Context,
         ContextExtension,
         Extracted,
         FailureDetail,
         Rejection,
+        Subject,
         describe,
         describeAsync,
         softCheck,