Add more text about TrueClass/FalseClass to schema modification doc

doc/schema_modification.rdoc

@@ -49,7 +49,8 @@ convert to database types:
   end
 
 Note that in addition to the ruby class name, Sequel also pays attention to the column options when
-determining which database type to use.
+determining which database type to use.  Also note that for boolean columns, you can use either
+TrueClass or FalseClass, they are treated the same way (ruby doesn't have a Boolean class).
 
 Also note that this conversion is only done if you use a supported ruby class name.  In all other
 cases, Sequel uses the type specified verbatim: