Removed test for unhandled case in function_item_references lint

Removed test for the unhandled case of calls to `fn f<T>(x: &T)` where `x` is a
function reference and is formatted as a pointer in `f`. This compiles since
`&T` implements `Pointer`, but is unlikely to occur in practice. Also tweaked
the lint's wording and modified tests accordingly.

compiler/rustc_mir/src/transform/function_references.rs

@@ -164,7 +164,7 @@ impl<'a, 'tcx> FunctionItemRefChecker<'a, 'tcx> {
         let ret = if fn_sig.output().skip_binder().is_unit() { "" } else { " -> _" };
         self.tcx.struct_span_lint_hir(FUNCTION_ITEM_REFERENCES, lint_root, span, |lint| {
             lint.build(&format!(
-                "cast `{}` with `as {}{}fn({}{}){}` to use it as a pointer",
+                "cast `{}` with `as {}{}fn({}{}){}` to obtain a function pointer",
                 ident,
                 unsafety,
                 abi,