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.
hum.. ok.. but let's remember to update the link to a tag in future, or think of a better way to not break such links.
Wouldn't it be better to just refer to the test name? Or if there is a specific relevant code snipped we usually include that in the docs.