message: add docs for `emulate_foreign_item_by_bame
diff: @@ -178,6 +178,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
  Ok(None)
  }
  + /// Emulates calling a foreign item using its name, failing if the item is not supported.
  + /// Returns Ok(false) if after calling this function, the call should return earlier instead of
  + /// going to the next block.
  fn emulate_foreign_item_by_name(
  &mut self,
  link_name: &str,
  