More mail notif capabilities doc and missing flags
More mail notif capabilities doc and missing flags

Added more documentation to capabilities, explaining the four possible
groups and giving examples of wich protocols support it. Also added missing
capabilities for Request methods and added a method to request a URL to
compose e-mail.

Signed-off-by: Nicolas Dufresne <36c35e0aac84fcb59b447cc6415df42e7fda8df7@collabora.co.uk>

freedesktop-unofficial-mirror/telepathy__telepathy-spec,TelepathyIM/telepathy-spec