minor #6936 Improved the title of Validation Groups article to make it easier to find (javiereguiluz)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #6936).

Discussion
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Improved the title of Validation Groups article to make it easier to find

This fixes #6782.

Commits
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8994b22 Improved the title of Validation Groups article to make it easier to find it

Author: weaverryan

validation/groups.rst

@@ -1,8 +1,8 @@
 .. index::
     single: Validation; Groups
 
-How to Apply only a Subset of all Your Validation Constraints
-=============================================================
+How to Apply only a Subset of all Your Validation Constraints (Validation Groups)
+=================================================================================
 
 By default, when validating an object all constraints of this class will
 be checked whether or not they actually pass. In some cases, however, you