doc: sha256 is no longer experimental

doc: sha256 is no longer experimental

Remove scary wording that basically stops people using sha256
repositories not because of interoperability issues with sha1
repositories, but from fear that their work will suddenly become
incompatible in some future version of git.

We should be clear that currently sha256 repositories will not work with
sha1 repositories but stop the scary words.

Signed-off-by: Adam Majer <adamm@zombino.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>