Add MSB3 to Variant Table

[kyzer-davis]

Split from #145, obviously this spec only cares about two-bits 1 and 0 but re-uses the table from RFC4122.
The final value of the "three bit variant" is don't care. thus 10x
However, the hex that gets converted is a 4 bit value. Thus later some text I wrote discussed the possible Hex values for the 10x variant. Which is really 10xx

I was reviewing the comment from OPSDIR and trying to figure out how to call out this 4th "don't care bit"...

(2) p 14, sec 4.2.  Version Field

      Table 2: UUID variant 10x versions defined by this specification
   An example version/variant layout for UUIDv4 follows the table where
   M represents the version placement for the hexadecimal representation
   of 0x4 (0b0100) and the N represents the variant placement for one of
   the four possible hexadecimal representation of variant 10x: 0x8
   (0b1000), 0x9 (0b1001), 0xA (0b1010), 0xB (0b1011)

Table 1 only defines 3 bits and doesn't obviously mention a 4th bit, and yet
the examples below are displaying 4 bits, where I assume that the 4th bit is
effectively arbitrary data in the UUID definition.  Please consider whether it
would be helpful to clarify this.

My solution

• Add another don't care bit to the table in MSB3 position (and call out the possible hex Values in a column as well.)
  • As far as I can see this wouldn't cause any problems as it does not change any values.

Thus:

{{table1}} lists the contents of the variant field, where the letter "x" indicates a "don't-care" value.

| Msb0 | Msb1 | Msb2 | Msb3 | Hex     |Description                                                                    |
|    0 |    x | x    | x    | 1-7     |Reserved, NCS backward compatibility and includes Nil UUID as per {{niluuid}}. |
|    1 |    0 | x    | x    | 8-9,A-B |The variant specified in this document.                                        |
|    1 |    1 | 0    | x    | C-D     |Reserved, Microsoft Corporation backward compatibility.                        |
|    1 |    1 | 1    | x    | E-F     |Reserved for future definition and includes Max UUID as per {{maxuuid}}.       |