Update AggregationExpression javadoc.

See: #4370

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationExpression.java

@@ -21,6 +21,10 @@ import org.springframework.data.mongodb.MongoExpression;
 /**
  * An {@link AggregationExpression} can be used with field expressions in aggregation pipeline stages like
  * {@code project} and {@code group}.
+ * <p>
+ * The {@link AggregationExpression expressions} {@link #toDocument(AggregationOperationContext)} method is called during
+ * the mapping process to obtain the mapped, ready to use representation that can be handed over to the driver as part
+ * of an {@link AggregationOperation pipleine stage}.
  *
  * @author Thomas Darimont
  * @author Oliver Gierke