libnm, docs: docs fixes

libnm, docs: docs fixes

Update the docs build to include and exclude the correct files.

Fill in some missing documentation, and fix problems in the existing
docs. (In particular, "<" can't appear as a literal in documentation,
so change it to "&lt;". Also, "PKCS#12" has to be written as
"PKCS#<!-- -->12", or gtk-doc will think "#12" is a reference to a
type named "12".)