Bluetooth: Document parallel scanner and initiator param selection

When scanning and connecting in parallel with
CONFIG_BT_SCAN_AND_INITIATE_IN_PARALLEL enabled, a Bluetooth controller
may choose different scheduling approaches based upon the provided
parameters.

Nordic's SoftDevice Controller performs better when both roles
use the same scan window and scan interval. To help customers
choosing the right configuration, this should be documented.

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>