Note that types do not have to be declared in closures

Without such a clarification, people who know and love closures (for instance
programmers with a Haskell background) might fear that types would have to
be declared in closures and that therefore using closures would be much more
unwieldy.

Author: iblech

src/doc/trpl/functions.md

@@ -75,7 +75,8 @@ This is a deliberate design decision. While full-program inference is possible,
 languages which have it, like Haskell, often suggest that documenting your
 types explicitly is a best-practice. We agree that forcing functions to declare
 types while allowing for inference inside of function bodies is a wonderful
-sweet spot between full inference and no inference.
+sweet spot between full inference and no inference. (For closures, i.e. unnamed
+functions, types do not have to be declared.)
 
 What about returning a value? Here's a function that adds one to an integer: