docs/cmdline: Rewrite the cpuid_mask_* section

docs/cmdline: Rewrite the cpuid_mask_* section

A large amount of the information here is obsolete since Xen 4.7

To being with, however, this patch marks a change in style for section
headings, due to how HTML anchors are generated.  Having more than one
parameter per heading makes an awkward anchor, especially when brace globbing
is used.  Furthermore, the misc suffixes such as (AMD only) get included, as
do the escaping for the underscores.

Markdown doesn't require escaped underscores in headings (I'm not entirely
sure how we ended up with that style), so remove them and fully expand the
glob syntax.  Also adjust com1,com2 while at it, which is the only other
multi-parameter heading.  Move the misc suffixes into an "Applicability:" note
alongside the information about defaults.

This results in the headings being unadorned, and identical to how they are
expressed on the command line and in code.

For cpuid_mask_cpu option, collapse the long line of almost identical strings
using [] globbing.  The result is much shorter and clearer to read.  Add a
warning that this option no longer masks all features on Fam15h and above, due
to not making use of the leaf 7 masks.

For the remainder of the cpuid_mask_* options, collapse them all together into
a single description.

Finally, leave an explicit note explaining that people should not be using
these options for migration safety.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>