Add dedicated auth-source section in ERC manual

Add dedicated auth-source section in ERC manual

* doc/misc/erc.texi: Move auth-source description from the Password
subheading of the Advanced chapter's Connecting section to the new
Integrations section as a new node, Auth-Source, and give it a bit
more structure.  Fix various misuses of xref vs. pxref.  Convert URL
subheading to subsection and add anchor.  Prefer "backend" as a single
word, based on usage in other manuals.  Also replace loud "warning" in
SASL troubleshooting section.
* etc/ERC-NEWS: Re-link auth-source mention.
* lisp/erc/erc-sasl.el (erc-sasl-auth-source-function): Update info
node in doc string.
* lisp/erc/erc-services.el (erc-auth-source-services-function):
Re-link auth-source info node in doc string.
* lisp/erc/erc.el (erc-password, erc-auth-source-server-function,
erc-auth-source-join-function): Re-link auth-source info node in doc
strings.