plan_cache_mode and postgresql.conf.sample

Hi,

While testing something that I needed to ensure a generic plan was
being used, during editing postgresql.conf I couldn't quite remember
the exact spelling of the option to do that.  I think the valid
options for the setting should be listed in the sample config file.

Patch attached.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services