docs, parallelism: Rearrange how jobserver reservations are made

docs, parallelism: Rearrange how jobserver reservations are made

Rasmus correctly observed that the existing jobserver reservation only
worked if no other build targets were specified. The correct approach
is to hold the jobserver slots until sphinx has finished. To fix this,
the following changes are made:

- refactor (and rename) scripts/jobserver-exec to set an environment
  variable for the maximally reserved jobserver slots and exec a
  child, to release the slots on exit.

- create Documentation/scripts/parallel-wrapper.sh which examines both
  $PARALLELISM and the detected "-jauto" logic from Documentation/Makefile
  to decide sphinx's final -j argument.

- chain these together in Documentation/Makefile

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/lkml/eb25959a-9ec4-3530-2031-d9d716b40b20@rasmusvillemoes.dk
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20191121205929.40371-4-keescook@chromium.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>