talkatu/talkatu

Fill out some missing documentation to get our coverage back to 100%

Bring the docs up to 100% coverage.

Testing Done:
built the docs verified that `build/talkatu/reference/talkatu-undocumented.txt` has 100% coverage. Also ran the unit tests and demo to make sure nothing inadvertantly got busted.

Reviewed at https://reviews.imfreedom.org/r/91/
if get_option('vapi')
if not get_option('introspection')
error('Vala generation requires GObject Introspection.')
endif
add_languages('vala')
talkatu_vapi = gnome.generate_vapi('talkatu',
sources : talkatu_gir[0],
packages : [ 'gtk+-3.0' ],
install : true,
gir_dirs : meson.current_build_dir() / '..' / 'talkatu',
)
endif # vala