Tweak look peg special docs

[sogaiu]

This PR contains suggested changes to the documentation for the look peg special and attempts to address #281.

For background please see the following issues:

• PEG look rule sometimes produces captures, despite documentation otherwise janet#1554
• Discrepancy between docs and behavior of look peg special #281

The changes include:

• Addressing the point made at the top of this comment:

  I don't know when it was decided that lookahead should not produce captures, but in most cases they do. We should fix the docs here.

• Simplifying the form of the signature and overall text as:

  (look ?offset patt)

  Matches only if patt matches at a fixed offset. offset can be any integer and defaults to 0. The peg will not advance any characters.

I find the new version shorter and clearer. I had found the previous version confusing but failed to think of a good way to improve things. Placing the ? before the offset label wasn't something that occurred to me and am grateful for that and the associated prose.

Thanks to @edsrzf for the original issue and the improved text 👍