Warn about implicit logging in erc-truncate-mode

Warn about implicit logging in erc-truncate-mode

* etc/ERC-NEWS: Add entry explaining that `erc-truncate-mode' no
longer quasi-activates `erc-log-mode' under certain conditions.
* lisp/erc/erc-log.el
(erc-log--call-when-logging-enabled-sans-module): Add helper for use
by the `truncate' module during initialization.
* lisp/erc/erc-truncate.el (erc-truncate-mode, erc-truncate-enable,
erc-truncate-disable): Warn on `erc-connect-pre-hook' when conditions
exist that would have seen logging transparently activated in older
ERC versions.
(erc-truncate--warn-about-logging): New function to warn about
implicit logging on pre-connect.
(erc-truncate-buffer-to-size): Clarify some comments and revise doc
string.
* test/lisp/erc/erc-scenarios-log.el (erc-scenarios-log--truncate):
Disable `erc-truncate-mode' even though `erc-modules' is shadowed so
that `erc-insert-done-hook' and friends are not contaminated.
`(Bug#60936)