[#2027] Backporting changes from @l3l0 on XML validation code

Author: weaverryan

reference/constraints/Blank.rst

@@ -43,6 +43,15 @@ of an ``Author`` class were blank, you could do the following:
             protected $firstName;
         }
 
+    .. code-block:: xml
+
+        <!-- src/Acme/BlogBundle/Resources/config/validation.xml -->
+        <class name="Acme\BlogBundle\Entity\Author">
+            <property name="firstName">
+                <constraint name="Blank" />
+            </property>
+        </class>
+
 Options
 -------
 

reference/constraints/MaxLength.rst

@@ -47,7 +47,7 @@ Basic Usage
         <class name="Acme\BlogBundle\Entity\Blog">
             <property name="summary">
                 <constraint name="MaxLength">
-                    <value>100</value>
+                    <option name="limit">100</option>
                 </constraint>
             </property>
         </class>

reference/constraints/NotBlank.rst

@@ -42,6 +42,15 @@ were not blank, you could do the following:
             protected $firstName;
         }
 
+    .. code-block:: xml
+
+        <!-- src/Acme/BlogBundle/Resources/config/validation.xml -->
+        <class name="Acme\BlogBundle\Entity\Author">
+            <property name="firstName">
+                <constraint name="NotBlank" />
+            </property>
+        </class>
+
 Options
 -------
 

reference/constraints/NotNull.rst

@@ -42,6 +42,15 @@ were not strictly equal to ``null``, you would:
             protected $firstName;
         }
 
+    .. code-block:: xml
+
+        <!-- src/Acme/BlogBundle/Resources/config/validation.xml -->
+        <class name="Acme\BlogBundle\Entity\Author">
+            <property name="firstName">
+                <constraint name="NotNull" />
+            </property>
+        </class>
+
 Options
 -------
 

reference/constraints/Null.rst

@@ -46,6 +46,15 @@ of an ``Author`` class exactly equal to ``null``, you could do the following:
             protected $firstName;
         }
 
+    .. code-block:: xml
+
+        <!-- src/Acme/BlogBundle/Resources/config/validation.xml -->
+        <class name="Acme\BlogBundle\Entity\Author">
+            <property name="firstName">
+                <constraint name="Null" />
+            </property>
+        </class>
+
 Options
 -------