[tiny doc fix] statistics are not retained across immediate shutdown

Hi,

In the following page, statistics are kept across server restarts:

http://www.postgresql.org/docs/current/static/monitoring-stats.html

"When the server shuts down, a permanent copy of the statistics data is stored in the global subdirectory, so that
statisticscan be retained across server restarts." 


However, statistics are not retained after immediate shutdown (pg_ctl stop -mi).  You may say "pg_ctl stop -mi is not a
shutdownbut an abort, so the sentence is not wrong", but it's an "immediate shutdown" and one mode of shutdown. 

I propose a tiny fix to clarify this.  Please find the attached patch.

I'd like this to be backported at least 9.2.  Thanks.


Regards, Takayuki Tsunakawa