doc: Update description of BT_RECV_IS_RX_THREAD.
doc: Update description of BT_RECV_IS_RX_THREAD.

Clarify why this option is needed and which restrictions it has if
disabled.

Signed-off-by: Joakim Andersson <cf0fcab5d38d6e132e92a97bcb0bae257abf4936@nordicsemi.no>

GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyriot/zephyr,kraj/zephyr,ldts/zephyr,kraj/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,explora26/zephyr,zephyriot/zephyr,finikorg/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,nashif/zephyr,kraj/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,kraj/zephyr,punitvara/zephyr,mbolivar/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,punitvara/zephyr,nashif/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyriot/zephyr,mbolivar/zephyr,explora26/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,ldts/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,nashif/zephyr,zephyriot/zephyr,galak/zephyr,galak/zephyr,galak/zephyr