Doc improvements for `avoid_as`.
Doc improvements for `avoid_as`.

Relevant conversation here: https://github.com/dart-lang/linter/commit/09f335118e39d233ae08ac60a717df559a73862c#commitcomment-14449921

BUG=
R=brianwilkerson@google.com

Review URL: https://codereview.chromium.org//1455183002 .

dart-lang/linter,alexeieleusis/dart-lang_linter,dart-lang/linter,alexeieleusis/dart-lang_linter