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.
> but the point is we should not persuade people to use this as a replacement for Hibernate Spatial

not anywhere I even mention Hibernate Spatial. I think it is pretty much obvious that we are talking about some new/additional Hibernate Search feature. After all it is the spatial _extension_. As you say, the original sentence was cumbersome. I prefer simple and concise sentences.

> While you improved readability, it lost the core of the message about combining the two aspects.

Not imo