[doc] ST_SetSRID should explain difference with ST_Transform

[komzpa]

A person claims they transformed their dataset into 3857 from 4326 and ST_DWithin "does not work". Upon investigation they used ST_SetSRID instead of ST_Transform. Our manual does not mention ST_Transform vs. ST_SetSRID on ST_SetSRID function manual.

[robe]

This looks like it's already addressed.

​http://postgis.net/docs/manual-3.0/ST_SetSRID.html

I see it in the note:

[Note]	

This function does not transform the geometry coordinates in any way - it simply sets the meta data defining the spatial reference system the geometry is assumed to be in. Use ST_Transform if you want to transform the geometry into a new projection.