doc: Remove reference to trigger file regarding promotion

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