Mention possibility of (limited!) type-checking of converter definitions.

pep-0437.txt

@@ -108,6 +108,10 @@ ampersand.
 Additional declarations may be placed into .c files. Duplicate declarations
 are allowed as long as the function types are identical.
 
+It is encouraged to declare custom converter types a second time right
+above the converter function definition. The preprocessor will then catch
+any mismatch between the declarations.
+
 
 TBD: Make a list of fantasy types like *rw_buffer*.