docs: In ecpg, clarify how username/password colon parameters are used

Backpatch to 9.2.

Patch from Alan B

doc/src/sgml/ecpg.sgml

@@ -194,9 +194,12 @@ EXEC SQL CONNECT TO unix:postgresql://sql.mydomain.com/mydb AS myconnection USER
 EXEC SQL BEGIN DECLARE SECTION;
 const char *target = "mydb@sql.mydomain.com";
 const char *user = "john";
+const char *passwd = "secret";
 EXEC SQL END DECLARE SECTION;
  ...
-EXEC SQL CONNECT TO :target USER :user;
+EXEC SQL CONNECT TO :target USER :user USING :passwd;
+
+EXEC SQL CONNECT TO :target USER :user/:passwd;
 </programlisting>
    The last form makes use of the variant referred to above as
    character variable reference.  You will see in later sections how C