Documentation: mention more worktree-specific exceptions

Documentation: mention more worktree-specific exceptions

If a directory in $GIT_DIR is overridden when $GIT_COMMON_DIR is set,
then usually all paths within that directory are overridden as well.
There are a couple of exceptions, though, and two of them, namely
'refs/rewritten' and 'logs/HEAD' are not mentioned in
'gitrepository-layout'.  Document them as well.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>