Bluetooth: Controller: Clarify that it can be overridden out of tree

Nordic devices are commonly used with the nRF Connect SDK.
There the SoftDevice Controller is set as the default
Bluetooth Controller. To avoid confusion when reading DTS
and Kconfig files, clarify this by adding a note.

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