_git: add help text for a few undocumented options (in git docs)

'# TODO's are left in place
_git: __git_files: do not pass $PREFIX to ls-files, breaks matcher-list

When getting file lists from git-ls-files, do not pass the
typed $PREFIX, because this breaks the matcher-list zstyle.

There is no need to pre-filter the results that strictly when
calling ls-files, beause the results are passed to _multi_parts
which deals with that anyways.

Example:

- Modifications to files dir/FIRST.txt and dir/SECOND.txt in a git repo
- matcher-list zstyle set to ignore case: m:{a-z}={A-Z}
- Attempt to complete: git checkout -- dir/f<tab>
- Before this fix: git-ls-files is called
    git ls-files -z --exclude-standard --modified -- 'dir/f*'
  and returns nothing, because it does not know of zsh's matcher-list.
  No completions are offered, even though matching should work
  case-insensitively.
- After this fix: git-ls-files is called
    git ls-files -z --exclude-standard --modified -- 'dir/*'
  which returns (dir/FIRST.txt dir/SECOND.txt).
  All results are then passed to _multi_parts which deals with the
  actual completion, taking into account the matcher spec,
  and correctly offers the matching result
    FIRST.txt