Documentation/git-update-ref.txt: discuss symbolic refs

Documentation/git-update-ref.txt: discuss symbolic refs

Add a paragraph which just emphasizes that the command without any
options does not support refs in the final arguments.  This is clear
already from the names `<new-oid>` and `<old-oid>` but the right balance
of redundancy makes documentation robust against stray interpretation.

This is also a good place to mention why `--stdin` has those `symref-*`
commands.

Suggested-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Taylor Blau <me@ttaylorr.com>