add usage notes for isNull

Author: zspitzer

docs/03.reference/01.functions/isnull/_usageNotes.md

@@ -0,0 +1,28 @@
+Always use scoped variable references with `isNull()`.
+
+When a null argument shares a name with an outer-scoped variable, `isNull()` can return the wrong result / unexpected due to scope cascading:
+
+```lucee
+name = "default";
+
+function greet( name ) {
+    if ( isNull( name ) ) {
+        writeOutput( "Hello stranger" );
+    } else {
+        writeOutput( "Hello #name#" );
+    }
+}
+
+greet( javacast( "null", "" ) );
+// Outputs: "Hello default" - found the outer variable, not the null argument!
+```
+
+Use scoped references to reliably check for null:
+
+```lucee
+if ( isNull( arguments.name ) ) { ... }
+if ( isNull( local.result ) ) { ... }
+if ( isNull( variables.config ) ) { ... }
+```
+
+See [[function-structkeyexists]] for additional information regarding null handling in CFML