add a comment about false positives from 'highlight renders' feature …

…in the React DevTools

src/react-render-perf/content.mdx

@@ -29,6 +29,9 @@ will appear with a cyan outline around it.
 
 <img src={highlightRerenders} width="500" />
 
+<em>NOTE: Sometimes the plugin with render a highlight rectangle around a component when
+it shouldn't so please double check that it's being rendered using the "⚛️ Profiler"</em>.
+
 Switch to the "Profiler" tab in the dialog and enable "Record why each component rendered while profiling".
 
 <img src={profilerSettingsUrl} width="500" />