Update manual on difference between nullable=false and optional=false

git-svn-id: https://svn.apache.org/repos/asf/openjpa/branches/2.0.x@939452 13f79535-47bb-0310-9956-ffa450edef68

openjpa-project/src/doc/manual/jpa_overview_mapping.xml

@@ -99,6 +99,14 @@ JPA uses the defaults defined by the specification. As we present
 each mapping throughout this chapter, we also describe the defaults that apply
 when the mapping is absent.
     </para>
+    <note>
+        <para>
+Mapping metadata is used primarily with schema generation. This metadata should not
+be relied upon for validation prior to communicating with the database. 
+For example using the @Column(nullable=false) annotation does not do up front validation
+that the value in the entity is correct. 
+        </para>
+    </note>
     <section id="jpa_overview_mapping_table">
         <title>
             Table

openjpa-project/src/doc/manual/jpa_overview_meta.xml

@@ -107,6 +107,15 @@ in the Reference Guide for details. There are currently no XML equivalents for
 these extension annotations.
         </para>
     </note>
+    <note>
+        <para>
+Persistence metadata may be used to validate the contents of your entities prior to communicating
+with the database. This differs from mapping meta data which is primarily used for schema generation.
+For example if you indicate that a relationship is not optional (e.g. @Basic(optional=false)) OpenJPA
+will validate that the variable in your entity is not null <emphasis>before</emphasis> inserting a row 
+in the database. 
+        </para>
+    </note>
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