Remove wrapping <div> in form helpers from doc

[ci skip]

89ff1f8 and 1de258e6 removed from the HTML generated by the form helpers
the <div> that was wrapping the field elements inside the <form>.

This commit updates the documentation of the methods to reflect the
two commits above.
Merge pull request #17001 from claudiob/remove-wrapping-div-from-form-helpers-doc

Remove wrapping <div> in form helpers from doc [ci skip]