Bluetooth: host: Document L2CAP channel send buffer sizes

Update the documentation for L2CAp Connection oriented channel send
function to include better description of the L2CAP PDUs (Basic frames)
and L2CAP SDUs (Credit-based frames) so that the application can better
understand how to size the buffer pools and setting the RX mtu.
Document stack behavior on RX path and how the application has to
set up the channel in order to receive segmented packets.
Document stack behavior on TX path for reserving either mandatory or
optional header bytes.

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