docs: clarify that --depth for git-fetch works with newly initialized repos

The original wording sounded as if --depth could only be used to deepen or
shorten the history of existing repos. However, that is not the case. In a
workflow like

    $ git init
    $ git remote add origin https://github.com/git/git.git
    $ git fetch --depth=1

The newly initialized repo is properly created as a shallow repo.

Signed-off-by: Junio C Hamano <gitster@pobox.com>