docs: warn against redirecting mailsync.sh into its own log

The obvious crontab line is the wrong one. The script writes to
~/.local/state/mailsync.log itself, so a caller appending to that same
file gets every line twice, which is what the first real run of the moved
script did.

Also records that rotation belongs to logrotate rather than the script,
and why: the two fought over the same file and the script overwrote a
compressed generation with an uncompressed one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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