pgsql: Mention pg_promote() as a method to trigger promotion in documen

Mention pg_promote() as a method to trigger promotion in documentation.

Previously in the "Standby Server Operation" section, pg_ctl promote and
protmote_trigger_file were documented as a method to trigger standby
promotion, but pg_promote() function not.

This commit also adds parentheses into <function>pg_promote</function>
in some docs to make it clearer that a function is being referred to.

Author: Masahiro Ikeda
Reviewed-by: Michael Paquier, Laurenz Albe, Tom Lane, Fujii Masao
Discussion: https://postgr.es/m/de0068417a9f4046bac693cbcc00bdc9@oss.nttdata.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/67f82e966b524fc0eb44024976c5178612a77fc8

Modified Files
--------------
doc/src/sgml/config.sgml | 2 +-
doc/src/sgml/high-availability.sgml | 7 ++++---
2 files changed, 5 insertions(+), 4 deletions(-)