rbd: import doesn't require image-spec arg, ditto for export and path

rbd: import doesn't require image-spec arg, ditto for export and path

Mark those as such in help and clarify what image-spec defaults to.

Related, all command args in our man page are enclosed into brackets.
I suppose the reason is that they are optional in the sense that you
can have commands like

    $ rbd clone --pool a --image b --snap -c --dest-pool d --dest e

with no args.  Given that we are trying to push people towards

    $ rbd clone a/b@c d/e

undo that so that real optional arguments can be marked optional.
While at it, add synopsis for each command and use backticks for
denoting commands more consistently.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>