git-remote-helpers.txt: document missing capabilities
git-remote-helpers.txt: document missing capabilities

Specifically, document the 'export' and '(im|ex)port-marks'
capabilities as well as the export command, which were
undocumented (but in active use).

Signed-off-by: Max Horn <0706025b2bbcec1ed8d64822f4eccd96314938d0@quendi.de>
Acked-by: Sverre Rabbelier <91e3618f486cb75969cf54d52cb06350cfe172f2@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git