Update contrib/unaccent documentation about its unaccent.rules file.

Commit 1bbd52cb9a didn't bother with such niceties.

doc/src/sgml/unaccent.sgml

@@ -55,7 +55,7 @@
 Ã        A
 Ä        A
 Å        A
-Æ        A
+Æ        AE
 </programlisting>
      The two characters must be separated by whitespace, and any leading or
      trailing whitespace on a line is ignored.
@@ -94,7 +94,10 @@
    A more complete example, which is directly useful for most European
    languages, can be found in <filename>unaccent.rules</>, which is installed
    in <filename>$SHAREDIR/tsearch_data/</> when the <filename>unaccent</>
-   module is installed.
+   module is installed.  This rules file translates characters with accents
+   to the same characters without accents, and it also expands ligatures
+   into the equivalent series of simple characters (for example, &AElig; to
+   AE).
   </para>
  </sect2>