`magit-remote-ref-format': cleanup docstring

- Reword the first line, and fix a typo ("pariticular" -> "particular").
- Single-quote symbols.
- Enumerate the possible values more concisely.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #915 from praet/remote-ref-format-doc

`magit-remote-ref-format': cleanup docstring