autobpm: Hide noisy '"BPM already exists" log

[JOJ0]

Description

When batch bpm computing, the plugin's output is very noisy. This change moves "BPM alredy exists for item" logs to DEBUG level.

• Documentation. (Added a note to the plugin docs Configuration section)
• Changelog.
• Tests.

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[snejus]

Hmm, my thinking behind making this an info log was that the plugin should report when it calculates the BPM or when it skips it at equal verbosity levels. Otherwise, a user may get confused why certain files matching their query are not 'picked up'.

[jackwilsdon]

I agree with @snejus here. fetchart and replaygain print out skipped items without -v. A quiet flag might be better here - fetchart already has this:

beets/beetsplug/fetchart.py

Lines 1522 to 1529 in 712bada

	cmd.parser.add_option(
	"-q",
	"--quiet",
	dest="quiet",
	action="store_true",
	default=False,
	help="quiet mode: do not output albums that already have artwork",
	)