Fix and update docs

GoaDocuments, GoaMaps, GoaPhotos, GoaExchange, GoaMediaServer,
GoaTicketing, GoaReadLater, and GoaPrinters.
doc: Add GOA_PROVIDER_GROUP_CHAT and GOA_PROVIDER_FEATURE_READ_LATER
Remove documentation from leaf-level provider classes

These were not set up with our documentation harness, are completely
hidden from the applications, and there isn't much value in mentioning
that the 'GoaOwncloudProvider is used to access ownCloud servers'.
Prepare 3.15.1