Add documentation for Geometry2d

Warning, you must merge #287 before this pr.
This pr contains the documentation of:
* `ST_Azimuth`
* `ST_GeometryTypeCode`
* `ST_MakeGrid`
* `ST_MakeGridPoints`
* `ST_MakePolygon`
* `ST_MinimumDiameter`
* `ST_RingBuffer`
* `ST_IsValidDetail`
* `ST_IsValidReason`
* `ST_OrderingEquals`
* `ST_Force2D`

see #290
@ebocher ST_RingBuffer doesn't allow `endcap=flat`?