Particles2D: Comment out section on direction

The parameter is not implemented in the gles3 renderer for now.
See https://github.com/godotengine/godot/issues/13492.

tutorials/2d/particle_systems_2d.rst

@@ -159,20 +159,24 @@ means particles are drawn according to their emission order (default).
 ParticlesMaterial settings
 --------------------------
 
-Direction
-~~~~~~~~~
+..
+
+    Commented out as not implemented in 3.x for now.
+
+    Direction
+    ~~~~~~~~~
 
-This is the base angle at which particles emit. Default is ``0`` (down):
+    This is the base angle at which particles emit. Default is ``0`` (down):
 
-.. image:: img/paranim1.gif
+    .. image:: img/paranim1.gif
 
-Changing it will change the emissor direction, but gravity will still
-affect them:
+    Changing it will change the emissor direction, but gravity will still
+    affect them:
 
-.. image:: img/paranim2.gif
+    .. image:: img/paranim2.gif
 
-This parameter is useful because, by rotating the node, gravity will
-also be rotated. Changing direction allows them to be separated.
+    This parameter is useful because, by rotating the node, gravity will
+    also be rotated. Changing direction allows them to be separated.
 
 Spread
 ~~~~~~