Loosen return type guidelines for local functions

Local named functions do infer a return type in current Dart versions.

In our recommendation to annotate the return type of functions change
the set of functions the guideline applies to from "named" functions to
"non-local" functions, anonymous function expressions are also local to
a method body.

Author: natebosch

src/content/effective-dart/design.md

@@ -1200,10 +1200,10 @@ makeGreeting(String who) {
 }
 ```
 
-Note that this guideline only applies to *named* function declarations:
-top-level functions, methods, and local functions. Anonymous function
-expressions infer a return type from their body. In fact, the syntax doesn't
-even allow a return type annotation.
+Note that this guideline only applies to *non-local* function declarations:
+top-level functions, and methods. Local functions and anonymous function
+expressions infer a return type from their body. In fact, the anonymous function
+syntax doesn't even allow a return type annotation.
 
 
 ### DO annotate parameter types on function declarations