Mention CREATE/DROP STATISTICS in event triggers docs

The new commands are reported by event triggers, but they weren't
documented as such.  Repair.

Author: David Rowley
Discussion: https://postgr.es/m/CAKJS1f-t-NE=AThB3zu1mKhdrm8PCb=++3e7x=Lf343xcrFHxQ@mail.gmail.com