Rollup merge of #21925 - sfackler:allow-missing-copy, r=alexcrichton

This was particularly helpful in the time just after OIBIT's
implementation to make sure things that were supposed to be Copy
continued to be, but it's now creates a lot of noise for types that
intentionally don't want to be Copy.

r? @alexcrichton

src/doc/reference.md

@@ -1813,7 +1813,6 @@ default visibility with the `priv` keyword. When an item is declared as `pub`,
 it can be thought of as being accessible to the outside world. For example:
 
 ```
-# #![allow(missing_copy_implementations)]
 # fn main() {}
 // Declare a private struct
 struct Foo;