docs: clarify file options in git-config `--edit`

docs: clarify file options in git-config `--edit`

The description for the `-e`/`--edit` option references scopes
inconsistently: system and global are referenced by their option name
(`--system`/`--global`), but repository (`--local` is not. Additionally,
neither `--worktree` nor `--file` are referenced at all, despite also
being a valid options.

Update the description to mention all four available scopes as well as
`--file`, referencing each consistently by their option name.

Signed-off-by: Brian Lyles <brianmlyles@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>