assert: update comments

Remove the numbers from the comments to make it clear that
assert does not follow the
[CJS spec](http://wiki.commonjs.org/wiki/Unit_Testing/1.0).
Additionally, clean up the existing comments for consistent
formatting/language and ease of reading.

PR-URL: https://github.com/nodejs/node/pull/10579
Fixes: https://github.com/nodejs/node/issues/9063


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>