net: dccp: Fix ccid2_rtt_estimator() kernel-doc

make C=1 reports:

warning: Function parameter or struct member 'mrtt' not described in 'ccid2_rtt_estimator'

So document the 'mrtt' parameter.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240505-ccid2_rtt_estimator-kdoc-v1-1-09231fcb9145@quicinc.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>