x86/virt/tdx: Document TDX module update

Recent changes introduced TDX module update support. This is a thorny
feature to use correctly. It is intended for informed admins only who are
expected to either be doing things very carefully, or using it via
userspace programs that handle the pitfalls for them.

Document the basics of how to use the feature and what is expected of the
user in order for it to go correctly. Both to help the intended users of
the feature and as a "here be dragons" note for the more casual TDX users.

[ dhansen: tweak docs a bit, clarify update constraints ]

Signed-off-by: Chao Gao <chao.gao@intel.com>
[dropped "Implementation details" section, update log]
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>

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