It is not documented that pg_promote can exit standby mode

Hi, 

The document(high-availability.sgml) says that there are only two ways 
to exit standby mode.

　26.2.2. Standby Server Operation
　Standby mode is exited and the server switches to normal operation when 
pg_ctl promote is run or a trigger file is found (promote_trigger_file).

But there is another way, by calling pg_promote function.
I think we need to document it, doesn't it?

I attached a patch. Please review and let me know your thoughts.

Regards,
Masahiro Ikeda