[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 statistics can 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 shutdown but 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