Bluetooth: host: Document not waiting for TX context from sys workqueue

Document the GATT APIs not waiting for TX context when being called
from the system workqueue context. This is because the TX contexts
are freed by the system workqueue so blocking would cause a deadlock.

The number of TX contexts are by default equal to the number of
TX buffers in the system. Since TX contexts is allocated after a buffer,
but freed from a lower priority than the buffer then there can be more
allocated TX contexts than TX buffers.

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