docs: Improve description of <hard_limit>

docs: Improve description of <hard_limit>

/domain/memtune/hard_limit provides a way to cap the memory a VM process
can use, including the amount of memory the process can lock. When memory
locking of a VM is requested, <hard_limit> can be used to prevent the
potential host DoS issue mentioned in /domain/memoryBacking/locked
description.

This patch improves the <hard_limit> text by clarifying it can be used
to prevent "host crashing" when VM memory is locked.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>