Simplify erc-sasl's auth-source API

Simplify erc-sasl's auth-source API

* doc/misc/erc.texi: Revise descriptions in SASL chapter to reflect
simplified auth-source options.

* lisp/erc/erc-sasl.el (erc-sasl-password,
erc-sasl-auth-source-function): Revise doc strings.
(erc-sasl-auth-source-password-as-host): New function to serve as
more useful choice for option `erc-sasl-auth-source-function'.
(erc-sasl--read-password): Promote auth-source to pole position, above
an explicit string and `:password'.

* test/lisp/erc/erc-sasl-tests.el (erc-sasl--read-password--basic):
Massage tests to conform to simplified `erc-sasl-password'
API.  (Bug#29108.)