RESOLVED - BATCH-699: Document thread safety

Added javadoc mostly to ItemReaders and ItemWriters adn fixed the misleading javadoc of ItemReader interface.

spring-batch-core/src/main/java/org/springframework/batch/core/BatchStatus.java

@@ -26,6 +26,8 @@ import java.io.Serializable;
  * A BatchStatus can be safely serialized, however, it should be noted that the pattern can break down if different
  * class loaders load the enumeration.
  * 
+ * This class is immutable and therefore thread-safe.
+ * 
  * @author Lucas Ward
  * @author Greg Kick
  */

spring-batch-core/src/main/java/org/springframework/batch/core/JobParameters.java

@@ -22,6 +22,8 @@ import org.apache.commons.lang.builder.HashCodeBuilder;
  * equals another. Furthermore, because these parameters will need to be
  * persisted, it is vital that the types added are restricted.
  * 
+ * This class is immutable and therefore thread-safe.
+ * 
  * @author Lucas Ward
  * @since 1.0
  */