clarify redaction_retention_period doc

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Description:
I came across this, it may apply to other files as well:

synapse/docs/sample_config.yaml

Lines 438 to 443 in 6660912

	# How long to keep redacted events in unredacted form in the database. After
	# this period redacted events get replaced with their redacted form in the DB.
	#
	# Defaults to `7d`. Set to `null` to disable.
	#
	#redaction_retention_period: 28d

please clarify in the doc whether disable means

• disabling the retention (time span) of unredacted event, replacing it immediately
• disabling the replacement of unredacted by redacted events, keeping unredacted indefinitely