[dartdev] update help text for 'dart doc'
[dartdev] update help text for 'dart doc'

Change-Id: Iac786b3dd471c4650e42aefacdddb177370f38f5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/227921
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Commit-Queue: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk