Update document/core/valid/matching.rst

Add note for seemingly identical premise and conclusion

Co-authored-by: Andreas Rossberg <[email protected]>

document/core/valid/matching.rst

@@ -591,6 +591,11 @@ A :ref:`tag type <syntax-tagtype>` :math:`\deftype_1` matches :math:`\deftype_2`
      C \vdashtagtypematch \deftype_1 \matchestagtype \deftype_2
    }
 
+.. note::
+   Although the conclusion of this rule looks identical to its premise,
+   they in fact describe different relations:
+   the premise invokes subtyping on defined types,
+   while the conclusion defines it on tag types that happen to be expressed as defined types.
 
 .. index:: external type, function type, table type, memory type, global type
 .. _match-externtype: