[mdoc] Add "internal" 'mdoc x-msitomsx' command.

[mdoc] Add "internal" 'mdoc x-msitomsx' command.

The 'mdoc x-msitomsx' command converts a Microsoft internal XML dialect into
Microsoft XML Documentation format (e.g. what 'mdoc export-msxdoc' generates).

The 'mdoc x-msitomsx' command is internal, i.e. 'mdoc help' won't list it,
as this format is completely internal to Microsoft (and likely subject to
change without notice); this command only exists to help facilitate the ECMA
standardization process, converting Microsoft's existing (internal)
documentation into a format usable by the ECMA committee.