Change return type of DOM\HTMLDocument::saveHTML() (#13701)

Strict error checking is always true for classes in "new DOM".
This means that we always throw an error when calling
`php_dom_throw_error`, and therefore the false return value is not
actually possible.
Also change the stub to reflect this.

Author: ndossche

ext/dom/html_document.c

@@ -1294,8 +1294,8 @@ PHP_METHOD(DOM_HTMLDocument, saveHTML)
 	if (nodep != NULL) {
 		DOM_GET_OBJ(node, nodep, xmlNodePtr, nodeobj);
 		if (node->doc != docp) {
-			php_dom_throw_error(WRONG_DOCUMENT_ERR, dom_get_strict_error(intern->document));
-			RETURN_FALSE;
+			php_dom_throw_error(WRONG_DOCUMENT_ERR, true);
+			RETURN_THROWS();
 		}
 	} else {
 		node = (const xmlNode *) docp;

ext/dom/php_dom.stub.php

@@ -1599,7 +1599,7 @@ public function saveXML(?Node $node = null, int $options = 0): string|false {}
         /** @implementation-alias DOMDocument::save */
         public function saveXMLFile(string $filename, int $options = 0): int|false {}
 
-        public function saveHTML(?Node $node = null): string|false {}
+        public function saveHTML(?Node $node = null): string {}
 
         public function saveHTMLFile(string $filename): int|false {}
     }