docs: fix log_line_prefix session id docs

Backpatch to 9.2.

Report from Ian Lawrence Barwick

doc/src/sgml/config.sgml

@@ -4001,7 +4001,7 @@ local0.*    /var/log/postgresql
          </informaltable>
 
          The <literal>%c</> escape prints a quasi-unique session identifier,
-         consisting of two 4-byte hexadecimal numbers (without leading zeros)
+         consisting of two hexadecimal numbers (without leading zeros)
          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