Document permissions needed for pg_database_size and pg_tablespace_size.

Back in 8.3, we installed permissions checks in these functions (see
commits 8bc225e799 and cc26599b72).  But we forgot to document that
anywhere in the user-facing docs; it did get mentioned in the 8.3 release
notes, but nobody's looking at that any more.  Per gripe from Suya Huang.

doc/src/sgml/func.sgml

@@ -13178,7 +13178,11 @@ postgres=# SELECT * FROM pg_xlogfile_name_offset(pg_stop_backup());
    <para>
     <function>pg_database_size</function> and <function>pg_tablespace_size</>
     accept the OID or name of a database or tablespace, and return the total
-    disk space used therein.
+    disk space used therein.  To use <function>pg_database_size</function>,
+    you must have <literal>CONNECT</> permission on the specified database
+    (which is granted by default).  To use <function>pg_tablespace_size</>,
+    you must have <literal>CREATE</> permission on the specified tablespace,
+    unless it is the default tablespace for the current database.
    </para>
 
    <para>