randomize_kstack: Improve docs on requirements/rationale

There were some recent questions about where and why to use the
random_kstack routines when applying them to new architectures[1].
Update the header comments to reflect the design choices for the
routines.

[1] https://lore.kernel.org/lkml/1652173338.7bltwybi0c.astroid@bobo.none



Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Kees Cook <keescook@chromium.org>