[TODO]: Improve POP3 docs

[polarathene]

Description

As detailed in this review comment, enabling POP3 will fail to retrieve any mail stored in junk (even if incorrectly flagged as spam).

For those not using Rspamd but enabling SpamAssassin, the SA_SPAM_SUBJECT ENV does not assist the POP3 situation due to the default behaviour to move mail to junk folders. This ENV should default to undef (avoids prepending anything to the mail subject header), optionally using the current default as an alternative fallback instead of undef only when related ENV to direct spam to a junk folder is explicitly disabled.

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Users interested in POP3 should be made aware of this implicit configuration gotcha regardless via an update to the POP3 docs.

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Another suggestion to improve UX is to catch this type of opt-in config scenario and emit warning log to raise awareness there:

This could be fixed by printing a warning if IMAP=0 and POP3=1 and MOVE_SPAM_TO_JUNK=1