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.
Ok, I change to:

"To make computations easier the latitude and longitude field values will be
projected into a flat coordinate system with the help of a http://en.wikipedia.org/wiki/Sinusoidal_projection[sinusoidal projection]."