x86/spec-ctrl: Improve all SPEC_CTRL_{ENTER,EXIT}_* comments

x86/spec-ctrl: Improve all SPEC_CTRL_{ENTER,EXIT}_* comments

... to better explain how they're used.

Doing so highlights that SPEC_CTRL_EXIT_TO_XEN is missing a VERW flush for the
corner case when e.g. an NMI hits late in an exit-to-guest path.

Leave a TODO, which will be addressed in subsequent patches which arrange for
VERW flushing to be safe within SPEC_CTRL_EXIT_TO_XEN.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 45f00557350dc7d0756551069803fc49c29184ca)