cpumask: limit visibility of FORCE_NR_CPUS

cpumask: limit visibility of FORCE_NR_CPUS

In current form, FORCE_NR_CPUS is visible to all users building their
kernels, even not experts.  It is also set in allmodconfig or
allyesconfig, which is not a correct behavior.

This patch fixes it.  It also changes the parameter short description:
removes implementation details and highlights the effect of the change.

Link: https://lkml.kernel.org/r/20221116172451.274938-1-yury.norov@gmail.com
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Cc: Alexey Klimov <klimov.linux@gmail.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Sander Vanheule <sander@svanheule.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>