A reminder that a block is a single expr in closures

This is something that's only been briefly mentioned in the beginning of
the tutorial and all of the closure examples within this subsection
include only one expression between { and }.

doc/tutorial.md

@@ -1439,10 +1439,15 @@ call_closure_with_ten(closure);
 ~~~~
 
 Closures begin with the argument list between vertical bars and are followed by
-a single expression. The types of the arguments are generally omitted,
-as is the return type, because the compiler can almost always infer
-them. In the rare case where the compiler needs assistance, though, the
-arguments and return types may be annotated.
+a single expression. Remember that a block, `{ <expr1>; <expr2>; ... }`, is
+considered a single expression: it evaluates to the result of the last
+expression it contains if that expression is not followed by a semicolon,
+otherwise the block evaluates to `()`.
+
+The types of the arguments are generally omitted, as is the return type,
+because the compiler can almost always infer them. In the rare case where the
+compiler needs assistance, though, the arguments and return types may be
+annotated.
 
 ~~~~
 let square = |x: int| -> uint { x * x as uint };