notes: correct documentation of DWIMery for notes references

notes: correct documentation of DWIMery for notes references

expand_notes_ref is used by --ref from git-notes(1) and --notes from the
git log to find the full refname of a notes reference. Previously the
documentation of these options was not clear about what sorts of
expansions would be performed. Fix the documentation to clearly and
accurately describe the behavior of the expansions.

Add a test for this expansion when using git notes get-ref in order to
prevent future patches from changing this behavior.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>