Update line about calldata use for external funcs

docs/types/reference-types.rst

@@ -28,6 +28,7 @@ annotation, the "data location", about where it is stored. There are three data
 ``memory``, ``storage`` and ``calldata``. Calldata is a non-modifiable,
 non-persistent area where function arguments are stored, and behaves mostly like memory.
 
+Calldata is usually preferred over memory for external functions but is not required after Solidity `0.6.9`.
 
 .. note::
     Prior to version 0.5.0 the data location could be omitted, and would default to different locations