Update look peg special docs

[sogaiu]

This PR contains changes to the docs for the look peg special.

The docs do not currently appear to cover the case of look being called with only a single argument:

(look offset patt)

Matches only if patt matches at a fixed offset. offset can be any integer. patt will not produce captures and the peg will not advance any characters.

This PR attempts to cover this additional case.

Please see here for some background.

[sogaiu]

Thanks to @pepe for discussion :+1: