LibWeb: Make it obvious that DOM::Document makes a copy of its source

Author: https://github.com/trflynn89
Commit: e7412717b4
Pull-request: https://github.com/SerenityOS/serenity/pull/15905
Reviewed-by: https://github.com/alimpfard ✅
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/DOM/Document.h

@@ -217,7 +217,7 @@ public:
     JS::NonnullGCPtr<HTMLCollection> all();
 
     String const& source() const { return m_source; }
-    void set_source(String const& source) { m_source = source; }
+    void set_source(String source) { m_source = move(source); }
 
     HTML::EnvironmentSettingsObject& relevant_settings_object();