Doc: Improve documentation of stream abort.

Doc: Improve documentation of stream abort.

Protocol v4 introduces parallel streaming, which allows Stream Abort
messages to include additional abort information such as LSN and
timestamp. However, the current documentation only states, "This field is
available since protocol version 4," which may misleadingly suggest that
the fields are always present when using protocol v4.

This patch clarifies that the abort LSN and timestamp are included only
when parallel streaming is enabled, even under protocol v4.

Author: Anthonin Bonnefoy <[email protected]>
Reviewed-by: Amit Kapila <[email protected]>
Backpatch-through: 16, where it was introduced
Discussion: https://postgr.es/m/CAO6_XqoKteQR1AnaR8iPcegbBE+HkAc2-g12rxN04yOt4-2ORg@mail.gmail.com