E0269: add suggestion to check for trailing semicolons

In situations where the value of the last expression must be inferred,
rustc will not emit the "you might need to remove the semicolon" warning,
so at least note this in the extended description.

Fixes: rust-lang#30497

src/librustc/diagnostics.rs

@@ -635,7 +635,17 @@ fn foo(x: u8) -> u8 {
 ```
 
 It is advisable to find out what the unhandled cases are and check for them,
-returning an appropriate value or panicking if necessary.
+returning an appropriate value or panicking if necessary. Check if you need
+to remove a semicolon from the last expression, like in this case:
+
+```ignore
+fn foo(x: u8) -> u8 {
+    inner(2*x + 1);
+}
+```
+
+The semicolon discards the return value of `inner`, instead of returning
+it from `foo`.
 "##,
 
 E0270: r##"