GTK-Doc description fields for tags can contain multiple lines and
even multiple paragraphs. Whitespace cannot be preserved in XML
attributes, so we move the "deprecated" description text into
a "<doc-deprecated />" element right next to where we already have
the "<doc />" element. Keep the "deprecated" attribute around for
backwards compatibility though, but set its value to "1" (analogous
to the "writable", "contruct", etc attributes) if the annotated
symbol is marked as deprecated.

While at it, add <doc-version /> and <doc-stability /> which
was not yet available in the .gir files...

This takes care of the "Since:", "Stability:" and "Deprecated:"
GTK-Doc tags. Nothing needs to be done for the "Returns:" tag as
as we already write a "<doc />" child element on "<return-value />".