doc: Add timeline as valid recovery target in standby.signal documentation

The documentation states that no target settings will be used when
standby.signal is present, but this is not quite the case since
recovery_target_timeline is a valid recovery target for a standby.

Update the documentation with this exception.

Author: David Steele <david@pgmasters.net>
Discussion: https://www.postgresql.org/message-id/flat/e445616d-023e-a268-8aa1-67b8b335340c%40pgmasters.net