build: Use AM_V_GEN where applicable
Fix and clean up docs

Removed GoaProviderFactory, GoaTelepathyFactory, GoaImapAuthLogin,
GoaSmtpAuth and utility functions from the public API documentation.
These are internal APIs that no application developer can or is meant
to use. Some of the text has been retained as internal documentation
for gnome-online-accounts hackers.

Fixed the goa_provider_get_all documentation and some cross-referencing
issues.
Prepare 3.15.3