Docs that quote crate doesn't support one-or-more repetition with plus + sign

[peter-lyons-kehl]

This is (I believe) more a problem/limitation in quote crate, but it affects proc-macro-rules' users.

* repetition compiles on both sides: proc-macro-rules and quote:

pub fn unsafe_fn(input: TokenStream) -> TokenStream {
    rules!(input.into() => {

        ( $f:expr => $( $arg:expr ),* ) => { quote! {
            #[deny(unused_unsafe)]
            unsafe {
                #f(
                    #(
                        #arg
                    ),*
                )
            }
        }}
}

But + repetition fails with a confusing error:

Of course, the developer can use + on rules! side, and * on quote side, but when one has narrow vision due to stress.... Suggest a note/example in the docs.