documenting signal flags

[Matthias Clasen <mclasen redhat com>]

Currently, gtk-doc ignores signal flags, but at least
the run first/run last/run cleanup information is vital
for anybody trying to use the signals. I have a patch
which makes gtk-doc emit the signal flags in much the
same way as it does the property flags now. 

Attached to http://bugzilla.gnome.org/show_bug.cgi?id=316789

Ok to commit ?

Matthias