doc/git-worktree: mention "refs/rewritten" as per-worktree refs

doc/git-worktree: mention "refs/rewritten" as per-worktree refs

Some references are special in the context of worktrees as they are
considered to be per-worktree instead of shared across all of the
worktrees. Most importantly, this includes "refs/worktree/" that have
explicitly been designed such that users can create per-woorktree refs.
But there are also special references that have an associated meaning
like "refs/bisect/", which is used to track state of git-bisect(1).

These special per-worktree references are documented in git-worktree(1),
but one instance is missing. In a9be29c9817 (sequencer: make refs
generated by the `label` command worktree-local, 2018-04-25), we have
converted "refs/rewritten/" to be a per-worktree reference as well.
These references are used by our sequencer infrastructure to generate
labels for rebased commits. So in order to allow for multiple concurrent
rebases to happen in different worktrees, these references need to be
tracked per worktree.

We forgot to update our documentation to mention these new per-worktree
references, which is fixed by this patch.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>