x86/srso: Explain the untraining sequences a bit more

stable inclusion
from stable-v5.10.192
commit d2be58f9215a480c1aed3e47e1b2b889d83a8fed
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I933RF

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2be58f9215a480c1aed3e47e1b2b889d83a8fed



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commit 9dbd23e4 upstream.

The goal is to eventually have a proper documentation about all this.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814164447.GFZNpZ/64H4lENIe94@fat_crate.local


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>