Rollup merge of rust-lang#35754 - QuietMisdreavus:must-use-reference,…

… r=Manishearth

Add `must_use` to the Reference

I'm a bit uncertain about the exact phrasing, but having it mentioned at all is probably better than before.

src/doc/reference.md

@@ -2071,6 +2071,9 @@ macro scope.
  trait of the same name. `{Self}` will be replaced with the type that is supposed
  to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate
  must be enabled.
+- `must_use` - on structs and enums, will warn if a value of this type isn't used or
+ assigned to a variable. You may also include an optional message by using
+ `#[must_use = "message"]` which will be given alongside the warning.
 
 ### Conditional compilation