Document ST_NumGeometries behaviour with empty geometries

[Mike Taves]

The docs for ​ST_NumGeometries do not describe the behavior with empty geometries. These currently return 0 for all empty geometry types, which is different than GEOS as shown in ​GEOS GH-949.

Xref #4736 and DevWikiEmptyGeometry

[robe]

I know this was a change trying to figure out why we did it. I think it was to resolve a crash issue.

[robe]

It looks to be described to me.

The ST_NumGeometries description says:

​https://postgis.net/docs/manual-3.4/ST_NumGeometries.html and

​https://postgis.net/docs/manual-dev/ST_NumGeometries.html

The description says:

"Returns the number of elements in a geometry collection (GEOMETRYCOLLECTION or MULTI*). For non-empty atomic geometries returns 1. For empty geometries returns 0. "

I do notice it's not described on the short description on this page:

​https://postgis.net/docs/manual-dev/ , which just says:

"Returns the number of elements in a geometry collection"

I feel like the added sentence on the front page to describe something people only accidentally use is not worth adding an additional sentence.