qdoc: Allow shared comments for global functions

Shared comments were only allowed for functions that
are members of classes. This update extends that
functionality to functions in namespaces, including
the global namespace.

Change-Id: Ida855d8b49c6cdfa1fe900d96af9841e05a87aee
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>