Reserve negative depth range for ERC's insert hooks

Reserve negative depth range for ERC's insert hooks

* etc/ERC-NEWS: Mention additional reserved depth range of -80 to -20.
Also mention possibly having to cycle module activation state after
updating options.
* lisp/erc/erc-goodies.el (erc-irccontrols-mode,
erc-irccontrols-enable): Add `erc-controls-highlight' to
`erc-insert-modify-hook' at depth -50.
* lisp/erc/erc.el (erc-insert-modify-hook): Mention negative
hook-depth range in doc string.

(cherry picked from commit d85f561da03cd4705341a5a73f5c643f778e0f35)