Unfortunately, the documentation has not been updated for a while (you can track progress here). Until this gets fixed, you can find documentation in header files or you can use help
command in Python:
>>> help(slicer.vtkMRMLMarkupsPlaneNode)
Help on class vtkMRMLMarkupsPlaneNode in module vtkSlicerMarkupsModuleMRMLPython:
class vtkMRMLMarkupsPlaneNode(vtkMRMLMarkupsNode)
 vtkMRMLMarkupsPlaneNode  MRML node to represent a plane markup

 Superclass: vtkMRMLMarkupsNode

 Plane Markups nodes contain three control points. Visualization
 parameters are set in the vtkMRMLMarkupsDisplayNode class.

 Markups is intended to be used for manual marking/editing of point
 positions.

 Coordinate systems used:
  Local: Local coordinates
  World: All parent transforms on node applied to local.
  Plane: Plane coordinate space (Origin of plane at 0,0,0, XYZ axis
 aligned to XYZ unit vectors). Can have additional offset/rotation
 compared to local.\ingroup Slicer_QtModules_Markups

 Method resolution order:
 vtkMRMLMarkupsPlaneNode
 vtkMRMLMarkupsNode
 MRMLCorePython.vtkMRMLDisplayableNode
 MRMLCorePython.vtkMRMLTransformableNode
 MRMLCorePython.vtkMRMLStorableNode
 MRMLCorePython.vtkMRMLNode
 vtkCommonCorePython.vtkObject
 vtkCommonCorePython.vtkObjectBase
 builtins.object

 Methods defined here:

 CopyContent(...)
 V.CopyContent(vtkMRMLNode, bool)
 C++: void CopyContent(vtkMRMLNode *node, bool deepCopy=true)
 override;

 Copy node content (excludes basic data, such as name and node
 references).
 \sa vtkMRMLNode::CopyContent

 CreateNodeInstance(...)
 V.CreateNodeInstance() > vtkMRMLNode
 C++: vtkMRMLNode *CreateNodeInstance() override;

 MRMLNode methods

 GetAutoSizeScalingFactor(...)
 V.GetAutoSizeScalingFactor() > float
 C++: virtual double GetAutoSizeScalingFactor()

 The plane size multiplier used to calculate the size of the
 plane. This is only used when the size mode is auto. Default is
 1.0.

 GetAxes(...)
 V.GetAxes([float, float, float], [float, float, float], [float,
 float, float])
 C++: void GetAxes(double x[3], double y[3], double z[3])

 The direction vectors defined by the markup points. Calculated as
 follows and then transformed by the offset matrix: X: Vector from
 1st to 0th point. Y: Cross product of the Z vector and X vectors.
 Z: Cross product of the X vector and the vector from the 2nd to
 0th point.

 GetAxesWorld(...)
 V.GetAxesWorld([float, float, float], [float, float, float],
 [float, float, float])
 C++: void GetAxesWorld(double x[3], double y[3], double z[3])

 GetClosestPointOnPlaneWorld(...)
 V.GetClosestPointOnPlaneWorld((float, float, float), [float,
 float, float], bool) > float
 C++: double GetClosestPointOnPlaneWorld(const double posWorld[3],
 double closestPosWorld[3], bool infinitePlane=true)

 Get the closest position on the plane in world coordinates.
 Returns the signed distance from the input point to the plane.
 Positive distance is in the direction of the plane normal, and
 negative distance is in the opposite direction.
 \param posWorld input position
 \param closestPosWorld: output found closest position
 \param infinitePlane if false, the closest position will be
 restricted to the plane bounds
 \return Signed distance from the point to the plane. Positive
 distance is in the direction of the plane normal

 GetIcon(...)
 V.GetIcon() > string
 C++: const char *GetIcon() override;

...