borg --help: list additional topics

[ThomasWaldmann]

besides the borg commands, we also have some additional help on some non-command topics.

see helptext dict in archiver.py.

it should be made clear in borg --help that help is available for these topics also, there should be a list of these topics.

See also #3088.

[ThomasWaldmann]

Note: this might require some argparse hacking.