HSEARCH-1716 Simplify the Lucene writing backend by dropping the parallel execution of writer tasks
HSEARCH-1273 Adding tip about default spatial field name org.hibernate.search.annotations.Spatial.COORDINATES_DEFAULT_FIELD.

Also reviewing and fine tuning rest of spatial chapter.
HSEARCH-1469 Javadocs and renaming of test class
HSEARCH-1469 Removing onDefaultCoordinates from SpatialContext and renaming onCoordinates to onField