Improve documentation for type-error form

[tov]

As a follow-up to #85, I’d like to document type-error a bit better as you requested. I’m adding this here as a reminder to do it myself.

I think it’s worth mentioning:

• Potential interaction between throwing an exception and syntax-parse’s backtracking.

• Treatment of the the parameters to the message format string.

Does that make sense? Should I also document the particular exception that gets thrown, exn:fail:type:check?

[tov]

(Can you assign this to me?)