glossary: clarify description of HEAD
glossary: clarify description of HEAD

HEAD on a branch does reference a commit via the branch ref it refers to.
The main difference of a detached HEAD is that it _directly_ refers to
a commit.  Clarify this.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git