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/visual_script/doc_classes/VisualScriptBuiltinFunc.xml

@@ -203,7 +203,7 @@
 			Deserialize a [Variant] from a [PoolByteArray] serialized using [constant VAR_TO_BYTES].
 		</constant>
 		<constant name="COLORN" value="63" enum="BuiltinFunc">
-			Return the [Color] with the given name and alpha ranging from 0 to 1
+			Return the [Color] with the given name and alpha ranging from 0 to 1.
 			[b]Note:[/b] Names are defined in [code]color_names.inc[/code].
 		</constant>
 		<constant name="MATH_SMOOTHSTEP" value="64" enum="BuiltinFunc">

modules/visual_script/doc_classes/VisualScriptEngineSingleton.xml

@@ -1,10 +1,10 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="VisualScriptEngineSingleton" inherits="VisualScriptNode" category="Core" version="3.2">
 	<brief_description>
-		A Visual Script node returning a singleton from [@GlobalScope]
+		A Visual Script node returning a singleton from [@GlobalScope].
 	</brief_description>
 	<description>
-		A Visual Script node returning a singleton from [@GlobalScope]
+		A Visual Script node returning a singleton from [@GlobalScope].
 	</description>
 	<tutorials>
 	</tutorials>

modules/visual_script/doc_classes/VisualScriptMathConstant.xml

@@ -21,28 +21,28 @@
 	</members>
 	<constants>
 		<constant name="MATH_CONSTANT_ONE" value="0" enum="MathConstant">
-			Unity: [code]1[/code]
+			Unity: [code]1[/code].
 		</constant>
 		<constant name="MATH_CONSTANT_PI" value="1" enum="MathConstant">
-			Pi: [code]3.141593[/code]
+			Pi: [code]3.141593[/code].
 		</constant>
 		<constant name="MATH_CONSTANT_HALF_PI" value="2" enum="MathConstant">
-			Pi divided by two: [code]1.570796[/code]
+			Pi divided by two: [code]1.570796[/code].
 		</constant>
 		<constant name="MATH_CONSTANT_TAU" value="3" enum="MathConstant">
-			Tau: [code]6.283185[/code]
+			Tau: [code]6.283185[/code].
 		</constant>
 		<constant name="MATH_CONSTANT_E" value="4" enum="MathConstant">
-			Mathematical constant [code]e[/code], the natural log base: [code]2.718282[/code]
+			Mathematical constant [code]e[/code], the natural log base: [code]2.718282[/code].
 		</constant>
 		<constant name="MATH_CONSTANT_SQRT2" value="5" enum="MathConstant">
-			Square root of two: [code]1.414214[/code]
+			Square root of two: [code]1.414214[/code].
 		</constant>
 		<constant name="MATH_CONSTANT_INF" value="6" enum="MathConstant">
-			Infinity: [code]inf[/code]
+			Infinity: [code]inf[/code].
 		</constant>
 		<constant name="MATH_CONSTANT_NAN" value="7" enum="MathConstant">
-			Not a number: [code]nan[/code]
+			Not a number: [code]nan[/code].
 		</constant>
 		<constant name="MATH_CONSTANT_MAX" value="8" enum="MathConstant">
 			Represents the size of the [enum MathConstant] enum.