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.
> To make computation

... easy? ...possible?
Something is missing. Should it be "To make it possible to compute.." ?
Not a new problem I see, but would be nice to fix it in this review.