minor #10948 Update sequence_provider.rst (DonCallisto)

This PR was submitted for the master branch but it was squashed and merged into the 3.4 branch instead (closes #10948).

Discussion
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Update sequence_provider.rst

I think that is better to explain this difference as someone could misunderstand and think that a group in the sequence, alone, will trigger the validation of all groups in the sequence.

Commits
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7f5d9c3 Update sequence_provider.rst

Author: javiereguiluz

validation/sequence_provider.rst

@@ -139,6 +139,13 @@ that group are valid, the second group, ``Strict``, will be validated.
     infinite recursion (as the ``Default`` group references the group
     sequence, which will contain the ``Default`` group which references the
     same group sequence, ...).
+    
+.. caution::
+    
+    Calling ``validate()`` with a group in the sequence (``Strict`` in previous example)
+    will cause a validation **only** with that group and not with all the groups in the 
+    sequence.
+    This is because sequence is now referred to ``Default`` group validation.
 
 You can also define a group sequence in the ``validation_groups`` form option::