enter_repo(): fix docs to match code

enter_repo(): fix docs to match code

In b3256eb (standardize and improve lookup rules for external local
repos), enter_repo() was modified to use a different precedence
ordering of suffixes for DWIM of the repository path, and to ensure
that the repository path is actually valid instead of just testing
for existence.

However, the documentation was not modified to reflect these
changes.  Fix the documentation to match the code.

Documentation contributed by Jeff King.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>