Fix regression in erc-nicks involving color pools

Fix regression in erc-nicks involving color pools

* lisp/erc/erc-nicks.el (erc-nicks-colors): Tweak doc.
(erc-nicks--create-pool-function): On graphic displays, set value to
`erc-nicks--create-culled-pool', the original default.  This fixes a bug
in which ERC mapped all pool members to a primary color, thus excluding
the bulk of them.  Thanks to Trevor Arjeski for discovering it.
(erc-nicks--create-coerced-pool): Mention the text-terminal-only
requirement in doc.
(erc-nicks-refresh): Improve doc.
* test/lisp/erc/erc-nicks-tests.el (erc-nicks-tests--track-faces):
Enable `erc-track-mode' prior to running body.

(cherry picked from commit c266c22b418541714889e85831de93c10a3c3fde)