Should document doc comment directives

[srawlins]

Right now, it looks like the only documentation for {@animation}, {@template}, {@youtube}, etc exists as doc comments on private methods in comment_processable.dart. We should document, perhaps in a top-level markdown file, or the wiki:

• the syntax for each directive
• any interoperability between them (can you write a {@template} with a {@youtube} inside?)
• what directives are accepted in Flutter's docs, pub.dev's docs, and Dart API docs

[srawlins]

Wiki page started at https://github.com/dart-lang/dartdoc/wiki/Doc-comment-directives

[NinaPacifier]

@srawlins Is this issue still open to working???

[kwalrath]

Once the page is a little more finished, we should refer to it from https://dart.dev/tools/dartdoc (and/or the pages it links to).