doc: Misc updates for AnimationNode* and others

- Add some missing descriptions.
- Add links to tutorials for ARVR and AnimationTree.
- Style fixes.
- Engine changes:
  * Make `AnimationNodeTransition.input_<number>` properties internal
    so that they don't appear in the docs. They still appear in the
    inspector based on the actual number of inputs requested.
  * Drop unimplemented `CPUParticles.flatness`. It's only used for 3D
    particles in `ParticlesMaterial`, and thus only relevant for
    `CPUParticles3D`.

doc/classes/VisualShaderNode.xml

@@ -7,6 +7,12 @@
 	<tutorials>
 	</tutorials>
 	<methods>
+		<method name="get_default_input_values" qualifiers="const">
+			<return type="Array">
+			</return>
+			<description>
+			</description>
+		</method>
 		<method name="get_input_port_default_value" qualifiers="const">
 			<return type="Variant">
 			</return>
@@ -15,6 +21,14 @@
 			<description>
 			</description>
 		</method>
+		<method name="set_default_input_values">
+			<return type="void">
+			</return>
+			<argument index="0" name="values" type="Array">
+			</argument>
+			<description>
+			</description>
+		</method>
 		<method name="set_input_port_default_value">
 			<return type="void">
 			</return>
@@ -27,8 +41,6 @@
 		</method>
 	</methods>
 	<members>
-		<member name="default_input_values" type="Array" setter="_set_default_input_values" getter="_get_default_input_values" default="[ 0, Vector3( 0, 0, 0 ) ]">
-		</member>
 		<member name="output_port_for_preview" type="int" setter="set_output_port_for_preview" getter="get_output_port_for_preview" default="-1">
 		</member>
 	</members>