Add documentation for Edit geometries function

see #231

In this PR there are documentation for: 
* `ST_AddZ`
* `ST_Densify`
* `ST_Interpolate3DLine`
* `ST_MultiplyZ`
* `ST_Normalize`
* `ST_RemoveHoles`
* `ST_RemovePoint`
* `ST_RemoveRepeatedPoints`
* `ST_Reverse`
* `ST_Reverse3DLine`
* `ST_UpdateZ`
* `ST_ZUpdateExtremities`