Documentation updates for v4.20/v5.0

Hello!

This series contains documentation updates. Some of these describe changes that are carried out in another branch, but are kept separate to allow reviewers to easily find the intent of those changes.

1. Add design documentation on interruption of NMI handlers.

2. Fix broken RCU-requirements link to LKML archive, courtesy of
Joel Fernandes.

3. Improve rcu_dynticks::dynticks documentation, courtesy of
Joel Fernandes.

4. Update documentation for removal of RCU-bh update machinery.

5. Fix broken HTML directive.

6. Update documentation for removal of RCU-sched update machinery.

Thanx, Paul

------------------------------------------------------------------------

 RCU/Design/Data-Structures/Data-Structures.html                 |    8 
 RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html |    9 
 RCU/Design/Requirements/Requirements.html                       |  164 ++--------
 RCU/stallwarn.txt                                               |   13 
 RCU/whatisRCU.txt                                               |    3 
 admin-guide/kernel-parameters.txt                               |   16 
 kernel-per-CPU-kthreads.txt                                     |    2 
 7 files changed, 73 insertions(+), 142 deletions(-)