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

While changing `magit-remote-ref-format's default to 'remote-slash-branch,
commit e7150a0a also corrected some mistakes in its docstring, but didn't
catch them all, and `magit-change-what-branch-tracks' also contained an
erroneous reference to `name-then-remote'.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-remote-ref-format' replace :version kw with :package-version

The ":version" keyword specifies in which version of *Emacs* the option
was introduced/changed.  We should use ":package-version" instead.

See (info "(elisp)Common Keywords").

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #908 from praet/remote-ref-format

Remote ref format