As indicated in WTForms documentation, fields have a description, which is
typically used for help text [1].

Also, in Twitter Bootstrap, help text are supported by `p.help-block` element
[2].

[1] http://wtforms.simplecodes.com/docs/1.0.1/fields.html#wtforms.fields.Field.__init__
[2] http://twitter.github.com/bootstrap/base-css.html#forms