[css-transforms-2] Add issue about formal syntax for SVG 3D transform functions.

Author: dbaron

css-transforms-2/Overview.bs

@@ -919,6 +919,8 @@ Three-dimensional transform functions and the properties 'perspective', 'perspec
 
 If the 'vector-effect' property is set to ''non-scaling-stroke'' and an object is within a <a>3D rendering context</a> the property has no affect on stroking the object.
 
+Issue: formally describe the syntax of the 3D transform functions in SVG,
+as is done <a href="https://drafts.csswg.org/css-transforms-1/#svg-syntax">for the 2-D functions</a>.
 
 The Transform Functions {#transform-functions}
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