Mention pg_reload_conf() in ALTER SYSTEM page

Hello,

I suggest mentioning pg_reload_conf() in the ALTER SYSTEM reference page as in the attached patch.  Considering the
administrationwith SQL, pg_reload_conf()would be preferred.  The following page also refers to pg_reload_conf(). 

http://www.postgresql.org/docs/devel/static/config-setting.html

Regards
Takayuki Tsunakawa