Add note about expression vs. statement to dining philosphers problem,

to address rust-lang#25488 .

src/doc/trpl/dining-philosophers.md

@@ -432,7 +432,9 @@ an extra annotation, `move`, to indicate that the closure is going to take
 ownership of the values it’s capturing. Primarily, the `p` variable of the
 `map` function.
 
-Inside the thread, all we do is call `eat()` on `p`.
+Inside the thread, all we do is call `eat()` on `p`. Also note that the call to `thread::spawn` lacks a trailing semicolon, making this an expression. This distinction is important, yielding the correct return value. For more details, read [Expressions vs. Statements][es].
+
+[es]: functions.html#expressions-vs.-statements
 
 ```rust,ignore
 }).collect();