Documentation/git-bundle.txt: mention full backup example

Documentation/git-bundle.txt: mention full backup example

Provide an example about how to make a “full backup” with caveats about
what that means in this case.

This is a requested use-case.[1]  But the doc is a bit unassuming
about it:

    If you want to match `git clone --mirror`, which would include your
    refs such as `refs/remotes/*`, use `--all`.

The user cannot be expected to formulate “I want a full backup” as “I
want to match `git clone --mirror`” for a bundle file or something.
Let’s drop this mention of `--all` later in the doc and frontload it.

† 1: E.g.:

    • https://stackoverflow.com/questions/5578270/fully-backup-a-git-repo
    • https://stackoverflow.com/questions/11792671/how-to-git-bundle-a-complete-repo

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>