Don't casemap erc-sasl-user when set to :nick

Don't casemap erc-sasl-user when set to :nick

* lisp/erc/erc-sasl.el (erc-sasl-user): Fix doc string.
(erc-sasl--get-user): Don't apply casemapping when the option
`erc-sasl-user' is set to `:nick'.  While many servers and
auth-services providers perform case-folding when comparing SASL
usernames, only some recognize RFC1459 mappings, which ERC previously
applied blindly.  Instead, ERC now behaves like other clients in
leaving such preparation in the hands of the server.  This bug was
introduced with changes new to ERC 5.5 and Emacs 29 (bug#29108).