add java doc

org.hl7.fhir.r5/src/main/java/org/hl7/fhir/r5/model/Base.java

@@ -258,32 +258,63 @@ public abstract class Base implements Serializable, IBase, IElement {
   }
 
 	// these 3 allow evaluation engines to get access to primitive values
+  
+  /**
+   * @return true if the data type is a primitive type and might have a primitive value 
+   *   (which will be accessed as a string, irrespective of the stated value)
+   */
 	public boolean isPrimitive() {
 		return false;
 	}
 	
+	/**
+	 * @return true if the type is boolean, and the primitive value can only be 'true' or 'false'
+	 */
   public boolean isBooleanPrimitive() {
     return false;
   }
 
+  /**
+   * @return true if the type is primitive, and there's value (e.g. no Data-Absent-Reason extension etc)
+   */
 	public boolean hasPrimitiveValue() {
 		return primitiveValue() != null;
 	}
 	
+	/**
+	 * @return the primitive value if there is one, as a string irrespective of the actual type (e.g. dates converted to their FHIR string representation)
+	 *    return null if the value is not a primitive or there is no value (might be extensions instead)
+	 */
 	public String primitiveValue() {
 		return null;
 	}
 	
+  /**
+   * @return true if the type is date|dateTime|instant, and the primitive value is a date/time of some precision
+   */
   public boolean isDateTime() {
     return false;
   }
 
+  /**
+   * @return the date/time value if there is one, or null
+   */
   public BaseDateTimeType dateTimeValue() {
     return null;
   }
-  
+
+  /**
+   * @return the FHIR type name of the instance (not the java class name)
+   */
 	public abstract String fhirType() ;
-	
+
+	/**
+	 * Note that this is potentially misleading on ElementDefinition that has a 'type' 
+	 * property - don't mistakenly use this thinking it's going to look at ElementDefinition.type
+   *
+	 * @param name - fhir type name
+	 * @return- true if it 'has' this type (including by specialization)
+	 */
 	public boolean hasType(String... name) {
 		String t = fhirType();
 		for (String n : name) {