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`.

modules/webrtc/doc_classes/WebRTCMultiplayer.xml

@@ -45,7 +45,7 @@
 			<return type="Dictionary">
 			</return>
 			<description>
-				Returns a dictionary which keys are the peer ids and values the peer representation as in [method get_peer]
+				Returns a dictionary which keys are the peer ids and values the peer representation as in [method get_peer].
 			</description>
 		</method>
 		<method name="has_peer">

modules/webrtc/doc_classes/WebRTCPeerConnection.xml

@@ -143,7 +143,7 @@
 			</argument>
 			<description>
 				Emitted when a new in-band channel is received, i.e. when the channel was created with [code]negotiated: false[/code] (default).
-				The object will be an instance of [WebRTCDataChannel]. You must keep a reference of it or it will be closed automatically. See [method create_data_channel]
+				The object will be an instance of [WebRTCDataChannel]. You must keep a reference of it or it will be closed automatically. See [method create_data_channel].
 			</description>
 		</signal>
 		<signal name="ice_candidate_created">