compiletest: Require `//~` annotations even if `error-pattern` is specified

src/tests/ui.md

@@ -335,6 +335,9 @@ But for strict testing, try to use the `ERROR` annotation as much as possible,
 including `//~?` annotations for diagnostics without span.
 For compile time diagnostics `error-pattern` should very rarely be necessary.
 
+Per-line annotations (`//~`) are still checked in tests using `error-pattern`,
+to opt out of these checks in exceptional cases use `//@ compile-flags: --error-format=human`.
+
 ### Error levels
 
 The error levels that you can have are: