Doc: reference the "stash list" in autostash docs

Doc: reference the "stash list" in autostash docs

In documentation pertaining to autostash behavior, we refer to the
"stash reflog". This description is too low-level as the reflog refers
to an implementation detail of how the stash works and, for end-users,
they do not need to be aware of this at all.

Change references of "stash reflog" to "stash list", which should
provide more accessible terminology for end-users.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>