Bluetooth: Controller: Rework FAKE_ENTROPY_NATIVE_POSIX text

Rework comment text for FAKE_ENTROPY_NATIVE_POSIX used as
entropy driver for the Controller on BOARD_NRF54L15BSIM.

Relates to commit 34b6b3d9 ("Bluetooth: Controller:
Support FAKE_ENTROPY_NATIVE_POSIX").

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>