Allow multi-character source strings in contrib/unaccent.

This could be useful in languages where diacritic signs are represented as
separate characters; more generally it supports using unaccent dictionaries
for substring substitutions beyond narrowly conceived "diacritic removal".
In any case, since the rule-file parser doesn't complain about
multi-character source strings, it behooves us to do something unsurprising
with them.

doc/src/sgml/unaccent.sgml

@@ -70,6 +70,14 @@
     </para>
    </listitem>
 
+   <listitem>
+    <para>
+     Actually, each <quote>character</> can be any string not containing
+     whitespace, so <filename>unaccent</> dictionaries could be used for
+     other sorts of substring substitutions besides diacritic removal.
+    </para>
+   </listitem>
+
    <listitem>
     <para>
      As with other <productname>PostgreSQL</> text search configuration files,