Mention `generateForAnnotatedDirectives` somewhere

[davidmorgan]

I realized ... well, I was told ;) dart-lang/build#4105 ... that we introduced generateForAnnotatedDirectives without mentioning it in CHANGELOG or docs.

Whoops.

Let's mention it somewhere?

[Rexios80]

Please also mention that if you are using TypeChecker().annotatedWith() you must now also check TypeChecker().libraryDirectivesAnnotatedWith().