netgraph/ng_bridge: Document staleness in multithreaded operation

In the data path of ng_bridge(4), the only value of the host struct,
which needs to be modified, is the staleness, which is reset every
time a frame is received. It's save to leave the code as it is.

This patch is part of a series to make ng_bridge(4) multithreaded.

Reviewed by: kp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D28546