Clarify description of incremental_skip_later

[jroitgrund]

Contrary to what the current docs say, incremental_skip_later should be set to no for beets to revisit skipped directories:

beets/beets/importer.py

Lines 582 to 586 in 3b78d93

	if session.config['incremental'] and not (
	# Should we skip recording to incremental list?
	self.skip and session.config['incremental_skip_later']
	):
	self.save_history()

[sampsyo]

A solid improvement; thank you!!