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>

path.c

@@ -301,14 +301,9 @@ return_null:
  * (3) "relative/path" to mean cwd relative directory; or
  * (4) "/absolute/path" to mean absolute directory.
  *
- * Unless "strict" is given, we try access() for existence of "%s.git/.git",
- * "%s/.git", "%s.git", "%s" in this order.  The first one that exists is
- * what we try.
- *
- * Second, we try chdir() to that.  Upon failure, we return NULL.
- *
- * Then, we try if the current directory is a valid git repository.
- * Upon failure, we return NULL.
+ * Unless "strict" is given, we check "%s/.git", "%s", "%s.git/.git", "%s.git"
+ * in this order. We select the first one that is a valid git repository, and
+ * chdir() to it. If none match, or we fail to chdir, we return NULL.
  *
  * If all goes well, we return the directory we used to chdir() (but
  * before ~user is expanded), avoiding getcwd() resolving symbolic