Clarify Guid.Variant property documentation (#12186)

* Initial plan

* Port runtime commit f05f25b2: Clarify Guid.Variant documentation

Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

* Refine wording to match runtime commit and style guide

Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>

* Update xml/System/Guid.xml

Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

* Update xml/System/Guid.xml

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jkotas <6668460+jkotas@users.noreply.github.com>
Co-authored-by: Jan Kotas <jkotas@microsoft.com>
Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

Author: 3 people

xml/System/Guid.xml

@@ -3140,8 +3140,8 @@ Because the `provider` parameter is ignored, you cannot use it to provide a cust
         <summary>Gets the value of the variant field for the <see cref="T:System.Guid" />.</summary>
         <value>To be added.</value>
         <remarks>
-          <para>This property corresponds to the most significant 4 bits of the 8th byte: 00000000-0000-0000-F000-000000000000. The "don't-care" bits are not masked out.</para>
-          <para>For more information on how to interpret this value, see <see href="https://www.rfc-editor.org/rfc/rfc9562.html">RFC 9562</see>.</para>
+          <para>This property returns all 4 bits as-is. If you only care about fewer bits of the variant field, see <see href="https://www.rfc-editor.org/rfc/rfc9562.html">RFC 9562</see> for information on how to interpret the result.</para>
+          <para>For example, [the UUIDv7 standard](https://www.rfc-editor.org/rfc/rfc9562.html#uuidv7) might only want to consider the two most significant bits of the field, as the least two significant bits are documented as "don't-care".</para>
         </remarks>
       </Docs>
     </Member>