document when functions were added

[Matthias Clasen <maclas gmx de>]

I have just committed support for a Since: tag in doc comments to
gtk-doc, and would now start to add this information for the functions
added in the 2.1 cycle. 

The question I have is how detailed this information should be. The
options are to 

a) use exact version numbers like 2.1.5 (would be quite a bit of work to
collect this information for all new symbols)

b) just use the next stable version: 2.2 (might look a bit funny if
"Since 2.2" shows up in the docs for 2.1.3, but I think it is
acceptable)

I would favour b).

Matthias