[mlir][doc] Update op optional-group asm format doc

[rkayaith]

Summary

Update docs to describe the behaviour when a default-valued attribute is
used as an optional-group's anchor after D134993.

mlir/docs/OpDefinitions.md

*   Exactly one argument variable or type directive within either
     -   The anchor is the element whose presence controls which elements
         should be printed/parsed.
     -   An element is marked as the anchor by adding a trailing `^`.
     -   The first element is *not* required to be the anchor of the group.
     -   When a non-variadic region anchors a group, the detector for printing
         the group is if the region is empty.
 *   Literals, variables, custom directives, and type directives are the only
     valid elements within the group.
-    -   Any attribute variable may be used, but only optional attributes can be
-        marked as the anchor.
+    -   Any attribute variable may be used, but only optional or default-valued
+        attributes can be marked as the anchor. A default-valued anchor is
+        considered present if it holds a value other than the default.
     -   Only variadic or optional results and operand arguments and can be used.
     -   All region variables can be used. When a non-variable length region is
         used, if the group is not present the region is empty.
 
 An example of an operation with an optional group is `func.return`, which has a
 variadic number of operands.
 
 ```tablegen