qdoc: Handle shared comments better

The shared comment in qdoc was originally meant only for
use with the \fn command, and only for multiple \fn commands
in the same class. But it proved to be useful for other things,
so people started using it in other contexts, even though it
didn't work there. This update should handle them all.

For example, when listing multiple \fn commands, they no longer
need be for functions in the same class. Also, multiple \typedef
commands are handled correctly.

Change-Id: I4be86026a227d74822f5f2295577adf0fe170d49
Reviewed-by: Martin Smith <martin.smith@qt.io>