message: syntax/doc: fix docs for try_case_fold_simple
  Its whole purpose is to not panic and instead return an error, which
  matches the implementation. This fixes the docs to properly reflect
  that.
diff: @@ -892,14 +892,11 @@ impl ClassUnicode {
  /// this class consists of the range `a-z`, then applying case folding will
  /// result in the class containing both the ranges `a-z` and `A-Z`.
  ///
  - /// # Panics
  + /// # Error
  ///
  - /// This routine panics when the case mapping data necessary for this
  - /// routine to complete is unavailable. This occurs when the `unicode-case`
  - /// feature is not enabled.
  - ///
  - /// Callers should prefer using `try_case_fold_simple` instead, which will
  - /// return an error instead of panicking.
  + /// This routine returns an error when the case mapping data necessary
  + /// for this routine to complete is unavailable. This occurs when the
  + /// `unicode-case` feature is not enabled.
  pub fn try_case_fold_simple(
  &mut self,
  ) -> result::Result<(), CaseFoldError> {
  