qdoc: Don't report error for things marked \internal

This change allows qdoc to avoid printing warnings about
a qdoc comment, if the comment contains the \internal
command. In these cases, the comment will not be
used in the documentation, so there is no point reporting
warnings about it. However, if the showinternal option is
used, warnings about comments marked internal are printed
anyway.

Change-Id: Idcb329958681523c79e9f6a3a144ae26d44a6906
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>