Add descriptive definitions of "slot" and "processor element" at the
top of the man page (and effectively delete / move some text from
lower in the man page up into those definitions).
Also add a little blurb in the --use-hwthread-cpus description about how
it changes the definition of "processor element".
This is not a cherry-pick from master because PRRTE has replaced ORTE
on master, and orterun.1in no longer exists in master.
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
(cherry picked from commit 07b8937d4ae8d64b6d2394f4272e705a2ec89656)
405dc6e7f2