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 <anthonin.bonnefoy@datadoghq.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 16, where it was introduced
Discussion: https://postgr.es/m/CAO6_XqoKteQR1AnaR8iPcegbBE+HkAc2-g12rxN04yOt4-2ORg@mail.gmail.com

doc/src/sgml/protocol.sgml

@@ -7292,8 +7292,8 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
        <term>Int64 (XLogRecPtr)</term>
        <listitem>
         <para>
-         The LSN of the abort. This field is available since protocol version
-         4.
+         The LSN of the abort operation, present only when streaming is set to parallel.
+         This field is available since protocol version 4.
         </para>
        </listitem>
       </varlistentry>
@@ -7302,9 +7302,9 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
        <term>Int64 (TimestampTz)</term>
        <listitem>
         <para>
-         Abort timestamp of the transaction. The value is in number
-         of microseconds since PostgreSQL epoch (2000-01-01). This field is
-         available since protocol version 4.
+         Abort timestamp of the transaction, present only when streaming is set to
+         parallel. The value is in number of microseconds since PostgreSQL epoch (2000-01-01).
+         This field is available since protocol version 4.
         </para>
        </listitem>
       </varlistentry>