OvmfPkg/PlatformPei: explain EFI_MEMORY_TYPE_INFORMATION page counts

OvmfPkg/PlatformPei: explain EFI_MEMORY_TYPE_INFORMATION page counts

Add a code comment that explains the nature of the NumberOfPages field
values. Including this kind of historical information was suggested by
Leif in <https://edk2.groups.io/g/devel/message/55797> (alternative link:
<http://mid.mail-archive.com/20200312104006.GB23627@bivouac.eciton.net>).

Right now, the most recent commit updating the page counts has been commit
991d95636264 ("[...] Update default memory type information to reduce EFI
Memory Map fragmentation.", 2010-07-16).

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Suggested-by: Leif Lindholm <leif@nuviainc.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20200312223555.29267-2-lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>