doc: Fix confusing description of streaming option in START_REPLICATION.

doc: Fix confusing description of streaming option in START_REPLICATION.

Previously, the documentation described the streaming option as a boolean,
which is outdated since it's no longer a boolean as of protocol version 4.
This could confuse users.

This commit updates the description to remove the "boolean" reference and
clearly list the valid values for the streaming option.

Back-patch to v16, where the streaming option changed to a non-boolean.

Author: Euler Taveira <euler@eulerto.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/8d21fb98-5c25-4dee-8387-e5a62b01ea7d@app.fastmail.com
Backpatch-through: 16