disallow asm! in #[naked] functions

also disallow the `noreturn` option, and infer `naked_asm!` as `!`

compiler/rustc_error_codes/src/error_codes/E0787.md

@@ -11,11 +11,10 @@ pub extern "C" fn f() -> u32 {
 }
 ```
 
-The naked functions must be defined using a single inline assembly
-block.
+The naked function must be defined using a single `naked_asm!` assembly block.
 
 The execution must never fall through past the end of the assembly
-code so the block must use `noreturn` option. The asm block can also
+code, so it must either return or diverge. The asm block can also
 use `att_syntax` and `raw` options, but others options are not allowed.
 
 The asm block must not contain any operands other than `const` and