minor #9978 [Validation] Added internal link (sir-kain)

This PR was squashed before being merged into the 2.8 branch (closes #9978).

Discussion
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[Validation]  Added internal link

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Adding the link to the API corresponding to the text "the API documentation
for the constraint "

Commits
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5c8c076 [Validation]  Added internal link

Author: javiereguiluz

validation.rst

@@ -477,7 +477,7 @@ options can be specified in this way.
 This is purely meant to make the configuration of the most common option of
 a constraint shorter and quicker.
 
-If you're ever unsure of how to specify an option, either check the API documentation
+If you're ever unsure of how to specify an option, either check :namespace:`Symfony\\Component\\Validator\\Constraints`
 for the constraint or play it safe by always passing in an array of options
 (the first method shown above).