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>

net/dccp/ccids/lib/tfrc_equation.c

@@ -688,6 +688,7 @@ u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
 
 /**
  * tfrc_invert_loss_event_rate  -  Compute p so that 10^6 corresponds to 100%
+ * @loss_event_rate: loss event rate to invert
  * When @loss_event_rate is large, there is a chance that p is truncated to 0.
  * To avoid re-entering slow-start in that case, we set p = TFRC_SMALLEST_P > 0.
  */