Bluetooth: Document reasons for HCI command timeouts

When reading the error message:
"ASSERTION_FAIL: command opcode 0x0c03 timeout with err -11" it may not be
obvious what is wrong with their setup unless you are very familiar
with HCI.

This commit adds some more documentation to make this more obvious.

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