designate "synthetic" class elements in "hh --show"

Summary: I've found it to be useful info
 * ::class is marked as ce_synthesized
 * DynamicYield is not, because we don't want overrides

Reviewed By: @jwatzman

Differential Revision: D1396749
remove normative "should" from <<override>> message

Summary: It's possible that the way to resolve the error is to remove
 the <<Override>> marking. "marked as" is more "neutral" than.

Reviewed By: @jwatzman

Differential Revision: D1396750