doc: Bluetooth: Document issue with certain controllers

Certain Qualcomm controllers do not accept our settings for Host Buffer
Size:

< HCI Command: Host Buffer Size (0x03|0x0033) plen 7       #19 [hci0]
22.391048
        ACL MTU: 27   ACL max packet: 6
        SCO MTU: 0    SCO max packet: 0
> HCI Event: Command Complete (0x0e) plen 4                #20 [hci0]
22.391525
      Host Buffer Size (0x03|0x0033) ncmd 1
        Status: Invalid HCI Command Parameters (0x12)

Likely due to the fact that we do not reserve space for any SCO packets
in the Host.

Other Controllers (Realtek) seem to not transmit any data at all in the
Controller to Host direction if Controller to Host flow control is
enabled.

Document this fact in the User Guide so that users know what to do when
this happens.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>