Do not replace undocumentable class synopses

Author: kocsismate

build/gen_stub.php

@@ -4299,6 +4299,9 @@ function replaceClassSynopses(string $targetDirectory, array $classMap, iterable
             $existingClassSynopses[$className] = $className;
 
             $classInfo = $classMap[$className];
+            if ($classInfo->isUndocumentable) {
+                continue;
+            }
 
             $newClassSynopsis = $classInfo->getClassSynopsisElement($doc, $classMap, $allConstInfos);
             if ($newClassSynopsis === null) {