Depart from normal man page proactice a little and provide guidance on
when to use assert, as well as providing a bad example of using
assert. While not strictly necessary, experience has shown issues
with poor assert choice happen often enough that this departure seems
warranted. Also, tighten up the previous example (there's no need
to have extra paragraphs or gratuitously long lines).

Reviewed by: emaste@ (earlier version)