giscanner: deprecate GObject-Introspection GTK-Doc tags

After discussing a different issue with the GTK-Doc maintainers,
we learned that our g-i specific top level tags should never have
existed in the first place. The prefered notation for annotations
that apply to the identifier should be written on the identifier
line, for example like what we already do with (skip).

As a result, this patch deprecates g-i specific top level tags and
implements them as annotations on the identifier instead but still
keeps support for malformed comment blocks using g-i specific top
level tags.

This means that all annotated code "out there" will continue to work
just fine with this version of g-i, but when a developer decides to
fix deprecation warnings in his/her comment blocks, the dependency
on g-i needs to be raised to a version that contains at least this
patch. #676133

https://bugzilla.gnome.org/show_bug.cgi?id=676133