Rollup merge of #119325 - RalfJung:custom-mir, r=compiler-errors

custom mir: make it clear what the return block is

Custom MIR recently got support for specifying the "unwind action", so now there's two things coming after the actual call part of `Call` terminators. That's not very self-explaining so I propose we change the syntax to imitate keyword arguments:
```
Call(popped = Vec::pop(v), ReturnTo(drop), UnwindContinue())
```

Also fix some outdated docs and add some docs to `Call` and `Drop`.

compiler/rustc_span/src/symbol.rs

@@ -1041,6 +1041,7 @@ symbols! {
         mir_offset,
         mir_retag,
         mir_return,
+        mir_return_to,
         mir_set_discriminant,
         mir_static,
         mir_static_mut,