Mention comments in Language Overview doc

[philrz]

A community zync user recently remarked:

I recently discovered we can use comments // in Zed query code. I guess I should have read the ZSON docs more throughly and figure this out sooner...

ZSON and the Zed language have some overlap but are in many ways wholly separate topics. I can see that comments are not currently mentioned at all in the Language Overview doc where they surely should be. In a quick scan my first instinct is to put it at the end of the Introduction right after it's shown how the language can extend from simple one-liner searches to longer source files that one might maintain in GitHub.