docs: log_line_prefix session id fix

Restore 4-byte designation for docs.  Fix 9.3 doc query to properly pad
to four digits.

Backpatch to all active branches

Per suggestions from Ian Lawrence Barwick

doc/src/sgml/config.sgml

@@ -4089,14 +4089,14 @@ local0.*    /var/log/postgresql
          </informaltable>
 
          The <literal>%c</> escape prints a quasi-unique session identifier,
-         consisting of two hexadecimal numbers separated by a dot.  The numbers
-         are the process start time and the
+         consisting of two 4-byte hexadecimal numbers separated by a dot.
+         The numbers are the process start time and the
          process ID, so <literal>%c</> can also be used as a space saving way
          of printing those items.  For example, to generate the session
          identifier from <literal>pg_stat_activity</>, use this query:
 <programlisting>
 SELECT to_hex(EXTRACT(EPOCH FROM backend_start)::integer) || '.' ||
-       to_hex(pid)
+       regexp_replace('0000' || to_hex(pid), '^0*(.*....)$', '\1')
 FROM pg_stat_activity;
 </programlisting>