Add documentation on nth and only-tags PEG specials

[katomuso]

No description provided.

[sogaiu]

I think nth has an optional tag argument...at least that's what it looks like to me here.

If that's the case, may be it's good to have that in the signature using ?tag or some such? FWIW, right below the proposed last change is uint which has ?tag at the end of its signature.

[katomuso]

I think nth has an optional tag argument...at least that's what it looks like to me here.

If that's the case, may be it's good to have that in the signature using ?tag or some such? FWIW, right below the proposed last change is uint which has ?tag at the end of its signature.

You are right, I've made the change.

[sogaiu]

Thanks for the update!