Let's break a long sentence into two shorter sentences

form/direct_submit.rst

@@ -91,7 +91,7 @@ method, pass the submitted data directly to
 .. caution::
 
     When the second parameter ``$clearMissing`` is ``false``, like with "PATCH"
-    method, the validation extension will only handle the submitted fields, if
+    method, the validation extension will only handle the submitted fields. If
     the underlying data needs to be validated, this should be done manually,
     i.e using the validator.