MdePkg: Add comments for SMBIOS Type 3 structure to cover SKU Number

MdePkg: Add comments for SMBIOS Type 3 structure to cover SKU Number

Starting with SMBIOS spec version 2.7, Type 3 added SKU Number.
SKU Number is at a variable offset (depends on count and length of Contained Elements), so cannot add SKU Number to the SMBIOS_TABLE_TYPE3 structure.
Adding comments to explain how to get SKU Number.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chris Phillips <chrisp@hpe.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>