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
No, we exclude the endcap flat.
For example ST_RingBuffer('LINESTRING(0 0, 1 1)', 3, 1, 'endcap=flat')
The function creates a polygon on each side instead of create one polygon. This behavior is not acceptable.