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/
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# Options
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option(
'nls',
type : 'boolean', value : true,
description : 'Install translation files'
)
option(
'doc',
type : 'boolean', value : true, yield : true,
description : 'build documentation with gtk-doc'
)
option(
'help2man',
type : 'boolean', value : true,
description : 'Whether or not to build man pages from --help output'
)
option(
'introspection',
type : 'boolean', value : true, yield : true,
description : 'build gobject-introspection support'
)
option(
'tests',
type : 'boolean', value : true,
description : 'run unit tests'
)
option(
'vapi',
type : 'boolean', value : true,
description : 'Whether or not to build vapi files for gplugin'
)