docs: fix log_line_prefix session id docs

Backpatch to 9.2.

Report from Ian Lawrence Barwick

doc/src/sgml/config.sgml

@@ -4089,8 +4089,8 @@ 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)
-         separated by a dot.  The numbers are the process start time and the
+         consisting of two 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: