Explain why tests/ui/ is unformatted.

Author: nnethercote

rustfmt.toml

@@ -19,7 +19,7 @@ ignore = [
     "/tests/run-make/translation/test.rs", # This test contains syntax errors.
     "/tests/rustdoc/",                # Some have syntax errors, some are whitespace-sensitive.
     "/tests/rustdoc-ui/",             # Some have syntax errors, some are whitespace-sensitive.
-    "/tests/ui/",
+    "/tests/ui/",                     # Some have syntax errors, some are whitespace-sensitive.
     "/tests/ui-fulldeps/",
 
     # Do not format submodules.