Document the tablespace directory "should" be empty, rather than "must"

be empty.  Because of binary migration usage, it might not be empty.

doc/src/sgml/ref/create_tablespace.sgml

@@ -81,7 +81,7 @@ CREATE TABLESPACE <replaceable class="parameter">tablespace_name</replaceable> [
       <listitem>
        <para>
         The directory that will be used for the tablespace. The directory
-        must be empty and must be owned by the
+        should be empty and must be owned by the
         <productname>PostgreSQL</> system user.  The directory must be
         specified by an absolute path name.
        </para>