[DOC] Fix "property can be set by constructor" hint printed too often

This hint should not be added to a property description if the object is
a singleton or has a private constructor.

Change-Id: I9b1e0791c7fb46d770eed3c6b4560c7a2918baef

tools/generate-doc.ts

@@ -733,7 +733,7 @@ class DocumentRenderer {
       }
       result.push(' mapped to this property.');
     }
-    if (property.const && !isStatic) {
+    if (property.const && !isStatic && !this.def.object && this.def.constructor.access === 'public') {
       result.push('\n\nThis property can only be set via constructor');
       if (this.def.isWidget || this.def.extends === 'Popup') {
         result.push(' or JSX');