doc: Remove reference to trigger file regarding promotion
The wording changed here comes from
991bfe11d28a, when the only way to
trigger a promotion was with a trigger file. There are more options to
achieve this operation these days, like the SQL function pg_promote() or
the command `pg_ctl promote`, so it is confusing to assume that only a
trigger file is able to do the work.
Note also that promote_trigger_file has been removed as of
cd4329d9393f
in 16~.
Author: Shinya Kato
Discussion: https://postgr.es/m/
201b08ea29aa61f96162080e75be503c@oss.nttdata.com
Backpatch-through: 12