[Form] improve placement of timestamp limitation notices

| Q             | A
| ------------- | ---
| Doc fix?      | rather an improvement
| New docs?     | no
| Applies to    | 2.1+
| Fixed tickets | none

 - Removes the notice for option `widget` on pages "`date` Field Type" and "`birthday` Field Type". It's already included by `date_input.rst.inc` and rendered for the `input` option.
 - Adds the notice to the "`datetime` Field Type" page for the `input` option.