Provide an example of expected PRI format

Previously, the PRI docs linked users to the
RFC for log formats. Although this is sufficient
for getting the required information, it is
not very convenient.

Understanding the correct format is required for
using this filter, so this information should be
included as a part of the documentation.

This patch adds that explanation along with an
example.
If you want good formatting, you'll want this indented with 5 spaces, not 3, and a blank line between "For Example:" and your example