Documentation/git-merge.txt: fix reference to synopsis

Documentation/git-merge.txt: fix reference to synopsis

437591a9d738 combined the synopsis of "The second syntax" (meaning `git
merge --abort`) and "The third syntax" (for `git merge --continue`) into
this single line:

       git merge (--continue | --abort | --quit)

but it was still referred to when describing the preconditions that have
to be fulfilled to run the respective actions. In other words:
References by number are no longer valid after a merge of some of the
synopses.

Also the previous version of the documentation did not acknowledge that
`--no-commit` would result in the precondition being fulfilled (thanks
to Elijah Newren and Junio C Hamano for pointing that out).

This change also groups `--abort` and `--continue` together when
explaining the prerequisites in order to avoid duplication.

Helped-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Michael Lohmann <mi.al.lohmann@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>