template0 database comment

People are confused about what template0 is for, so I created the
attached one-line patch to add a database comment to template0. No
initdb, I assume, becuase it is just a comment.

I plan to work on more system table and view comments for 9.2.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +