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().

(cherry picked from commit 0aa35289b3b51e09a5757c9991212ec416d281f2)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>