HSEARCH-1273 Adding tip about default spatial field name org.hibernate.s...

...earch.annotations.Spatial.COORDINATES_DEFAULT_FIELD.

Also reviewing and fine tuning rest of spatial chapter.
not sure.  i am not a big fan of referring to test classes and in this case even methods at all. I would not do that. But since it is there, we might as well link to it. IMO the example should be in the documentation itself.