CLEANUP: h3: fix documentation of h3_rcv_buf()

CLEANUP: h3: fix documentation of h3_rcv_buf()

Return value of h3_rcv_buf() is incorrectly documented. Indeed, it may
return a positive value to indicate that input bytes were converted into
HTX. This is especially important, as caller uses this value to consume
the reported data amount in QCS Rx buffer.

This should be backported up to 2.6. Note that on 2.8, h3_rcv_buf() was
named h3_decode_qcs().