adjust the dartdev help text for language experiments
adjust the dartdev help text for language experiments

Change-Id: I869a246d989ab41cba0619873de141dab6ebb280
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/160140
Reviewed-by: Jaime Wren <cd1ab83d11dd618c6b8cd01592020d0bd5183fdc@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