Documentation: rw_lock lessons learned

In recent months, two different network projects erroneously
strayed down the rw_lock path.  Update the Documentation
based upon comments by Eric Dumazet and Paul E. McKenney in
those threads.

Further updates await somebody else with more expertise.

Changes:
  - Merged with extensive content by Stephen Hemminger.
  - Fix one of the comments by Linus Torvalds.

Signed-off-by:  <William.Allen.Simpson@gmail.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>