Update source functions - include variable scoping [DOC-1065].md

Author: sade-wusi

src/connections/functions/source-functions.md

@@ -261,6 +261,12 @@ The `Segment.set()` method accepts an object with the following fields:
 > warning ""
 > When you use the `set()` method, you won't see events in the Source Debugger. Segment only sends events to connected warehouses.
 
+### Variable scoping
+
+When declaring settings variables, make sure to declare them in the function handler rather than globally in your Function. This prevents you leaking the settings values across other function instances. 
+
+The handler for Source functions is `onRequest()`.
+
 ### Runtime and dependencies
 
 {% include content/functions/runtime.md %}