Add description and images for ST_Union

Add description and images for ST_Union.

Just saw that there was already a Unit Test here https://github.com/irstv/H2GIS/blob/2bb4a01ea0f5cc02f17ceeaf736baf9a0c0d90c9/h2spatial/src/test/java/org/h2gis/h2spatial/SpatialFunctionTest.java

Maybe we can update the test (line 96) ?