The (original) section on "Operators and Delimiters" introduced
superfluous terminology ("delimiter", "special token") which
didn't matter and was used inconsistently.

Removed any mention of "delimiter" or "special token" and now
simply group the special character tokens into "operators"
(clearly defined via links), and "punctuation" (everything else).

Fixes #19450.

Change-Id: Ife31b24b95167ace096f93ed180b7eae41c66808
Reviewed-on: https://go-review.googlesource.com/38073Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>