Documentation updates to 'git-update-ref(1)'.
* kh/update-ref:
Documentation: mutually link update-ref and symbolic-ref
Documentation/git-update-ref.txt: discuss symbolic refs
Documentation/git-update-ref.txt: remove confusing paragraph
Documentation/git-update-ref.txt: demote symlink to last section
Documentation/git-update-ref.txt: remove safety paragraphs
Documentation/git-update-ref.txt: drop “flag”