Bluetooth: Mesh: Clean up API documentation

Fills in all missing API documentation for the public Bluetooth Mesh
API, and unifies the formatting of all entries.

Some defines and enum values have been left without documentation in
cases where the meaning is self explanatory, and documentation wouldn't
add any value. Structure members that are just internal parameters are
left without documentation, as this also hides them from the generated
documentation output.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>