cache.h: clarify documentation for git_path, et al

cache.h: clarify documentation for git_path, et al

The comment above these functions actually describes
sha1_file_name, and comes from the very first revision of
git. Commit 723c31f (Add "git_path()" and "head_ref()"
helper functions., 2005-07-05) added git_path, pushing the
comment away from the function it describes; later commits
added more functions in this block.

Let's fix the comment to describe these related functions in
more detail. Let's also make sure to point out their safer
alternatives (and move those alternatives below, which makes
more sense when reading the file).

Note that we do not need to move the existing comment to
sha1_file_name.  Commit d40d535 (sha1_file.c: document a
bunch of functions defined in the file, 2014-02-21) already
added a much more descriptive comment to it.

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