dccp: tfrc: fix doc warnings in tfrc_equation.c

Add description for `tfrc_invert_loss_event_rate` to fix the W=1 warnings:

 net/dccp/ccids/lib/tfrc_equation.c:695: warning: Function parameter or
  member 'loss_event_rate' not described in 'tfrc_invert_loss_event_rate'

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Richard Sailer <richard_siegfried@systemli.org>
Signed-off-by: David S. Miller <davem@davemloft.net>