Explain why we don't run rustfmt on tests/crashes/.

Author: nnethercote

rustfmt.toml

@@ -15,7 +15,7 @@ ignore = [
     # - some UI tests are broken by different formatting
     # - some require special comments in a particular position (e.g. `EMIT_MIR` comments)
     "/tests/codegen/simd-intrinsic/", # Many types like `u8x64` are better hand-formatted.
-    "/tests/crashes/",
+    "/tests/crashes/",                # Many tests contain syntax errors.
     "/tests/debuginfo/",
     "/tests/incremental/",
     "/tests/mir-opt/",