minor #8103 Update form best practises regarding Form::isValid (paulferrett)

This PR was submitted for the 3.3 branch but it was merged into the 3.2 branch instead (closes #8103).

Discussion
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Update form best practises regarding Form::isValid

This small change updates the form best practises to reflect the fact that calling `Form::isValid()` with an unsubmitted form is deprecated since version 3.2 and will throw an exception in 4.0.

Commits
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e4ec7bd Update form best practises regarding Form::isValid

Author: xabbuh

best_practices/forms.rst

@@ -204,7 +204,6 @@ and a ``createAction()`` that *only* processes the form submit. Both those
 actions will be almost identical. So it's much simpler to let ``newAction()``
 handle everything.
 
-Second, we recommend using ``$form->isSubmitted()`` in the ``if`` statement
-for clarity. This isn't technically needed, since ``isValid()`` first calls
-``isSubmitted()``. But without this, the flow doesn't read well as it *looks*
-like the form is *always* processed (even on the GET request).
+Second, is it required to call ``$form->isSubmitted()`` in the ``if`` statement
+before calling ``isValid()``. Calling ``isValid()`` with an unsubmitted form
+is deprecated since version 3.2 and will throw an exception in 4.0.