message: less unsupported errors in Miri, and clarifying comments
diff: @@ -229,13 +229,13 @@ pub fn report_error<'tcx, 'mir>(
  Unsupported(
  UnsupportedOpInfo::ThreadLocalStatic(_) |
  UnsupportedOpInfo::ReadExternStatic(_) |
  - UnsupportedOpInfo::PartialPointerOverwrite(_) | // we make memory uninit instead
  + UnsupportedOpInfo::PartialPointerOverwrite(_) |
  + UnsupportedOpInfo::PartialPointerCopy(_) |
  UnsupportedOpInfo::ReadPointerAsBytes
  ) =>
  panic!("Error should never be raised by Miri: {kind:?}", kind = e.kind()),
  Unsupported(
  - UnsupportedOpInfo::Unsupported(_) |
  - UnsupportedOpInfo::PartialPointerCopy(_)
  + UnsupportedOpInfo::Unsupported(_)
  ) =>
  vec![(None, format!("this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support"))],
  UndefinedBehavior(UndefinedBehaviorInfo::AlignmentCheckFailed { .. })
  