HSEARCH-1384 Adding the ability to set the number of entity types to
index in parallel in the Mass Indexer.
HSEARCH-1384 Updating the MassIndexer documentation
Merge branch 'master' into HSEARCH-1387

Conflicts:
	orm/src/main/java/org/hibernate/search/impl/MassIndexerImpl.java
HSEARCH-1387 Taking HSEARCH-1384 into account