filled out docs for various mesh types

(cherry picked from commit ef45f5ce75)

doc/classes/ArrayMesh.xml

@@ -1,6 +1,7 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="ArrayMesh" inherits="Mesh" category="Core" version="3.1.2">
 	<brief_description>
+		[Mesh] type that provides utility for constructing a surface from arrays.
 	</brief_description>
 	<description>
 		The [code]ArrayMesh[/code] is used to construct a [Mesh] by specifying the attributes as arrays. The most basic example is the creation of a single triangle
@@ -30,6 +31,7 @@
 			<argument index="0" name="name" type="String">
 			</argument>
 			<description>
+				Add name for a blend shape that will be added with [method add_surface_from_arrays]. Must be called before surface is added.
 			</description>
 		</method>
 		<method name="add_surface_from_arrays">
@@ -187,6 +189,7 @@
 			<argument index="2" name="data" type="PoolByteArray">
 			</argument>
 			<description>
+				Updates a specified region of mesh arrays on GPU. Warning: only use if you know what you are doing. You can easily cause crashes by calling this function with improper arguments.
 			</description>
 		</method>
 	</methods>
@@ -194,7 +197,7 @@
 		<member name="blend_shape_mode" type="int" setter="set_blend_shape_mode" getter="get_blend_shape_mode" enum="Mesh.BlendShapeMode">
 		</member>
 		<member name="custom_aabb" type="AABB" setter="set_custom_aabb" getter="get_custom_aabb">
-			An overriding bounding box for this mesh.
+			Overrides the [AABB] with one defined by user for use with frustum culling. Especially useful to avoid unnexpected culling when using a shader to offset vertices.
 		</member>
 	</members>
 	<constants>

doc/classes/MeshInstance.xml

@@ -43,6 +43,7 @@
 			<return type="int">
 			</return>
 			<description>
+				Returns the number of surface materials.
 			</description>
 		</method>
 		<method name="set_surface_material">

doc/classes/MeshInstance2D.xml

@@ -1,8 +1,10 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="MeshInstance2D" inherits="Node2D" category="Core" version="3.1.2">
 	<brief_description>
+		Node used for displaying a [Mesh] in 2D.
 	</brief_description>
 	<description>
+		Node used for displaying a [Mesh] in 2D. Can be constructed from an existing [Sprite] use tool in Toolbar. Select "Sprite" then "Convert to Mesh2D", select settings in popup and press "Create Mesh2D".
 	</description>
 	<tutorials>
 		<link>http://docs.godotengine.org/en/3.1/tutorials/2d/2d_meshes.html</link>
@@ -11,10 +13,13 @@
 	</methods>
 	<members>
 		<member name="mesh" type="Mesh" setter="set_mesh" getter="get_mesh">
+			The [Mesh] that will be drawn by the [MeshInstance2D].
 		</member>
 		<member name="normal_map" type="Texture" setter="set_normal_map" getter="get_normal_map">
+			The normal map that will be used if using the default [CanvasItemMaterial].
 		</member>
 		<member name="texture" type="Texture" setter="set_texture" getter="get_texture">
+			The [Texture] that will be used if using the default [CanvasItemMaterial]. Can be accessed as [code]TEXTURE[/code] in CanvasItem shader.
 		</member>
 	</members>
 	<constants>

doc/classes/PrimitiveMesh.xml

@@ -4,7 +4,7 @@
 		Base class for all primitive meshes. Handles applying a [Material] to a primitive mesh.
 	</brief_description>
 	<description>
-		Base class for all primitive meshes. Handles applying a [Material] to a primitive mesh.
+		Base class for all primitive meshes. Handles applying a [Material] to a primitive mesh. Examples include [CapsuleMesh], [CubeMesh], [CylinderMesh], [PlaneMesh], [PrismMesh], [QuadMesh], and [SphereMesh].
 	</description>
 	<tutorials>
 	</tutorials>
@@ -13,13 +13,16 @@
 			<return type="Array">
 			</return>
 			<description>
+				Returns mesh arrays used to constitute surface of [Mesh]. Mesh array can be used with [ArrayMesh] to create new surface.
 			</description>
 		</method>
 	</methods>
 	<members>
 		<member name="custom_aabb" type="AABB" setter="set_custom_aabb" getter="get_custom_aabb">
+			Overrides the [AABB] with one defined by user for use with frustum culling. Especially useful to avoid unnexpected culling when  using a shader to offset vertices.
 		</member>
 		<member name="flip_faces" type="bool" setter="set_flip_faces" getter="get_flip_faces">
+			If set, the order of the vertices in each triangle are reversed resulting in the backside of the mesh being drawn. Result is the same as using *CULL_BACK* in [SpatialMaterial]. Default is false.
 		</member>
 		<member name="material" type="Material" setter="set_material" getter="get_material">
 			The current [Material] of the primitive mesh.

doc/classes/QuadMesh.xml

@@ -12,6 +12,7 @@
 	</methods>
 	<members>
 		<member name="size" type="Vector2" setter="set_size" getter="get_size">
+			Size in the X and Y axes. Default is [code]Vector2(1, 1)[/code].
 		</member>
 	</members>
 	<constants>

doc/classes/TriangleMesh.xml

@@ -1,8 +1,10 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="TriangleMesh" inherits="Reference" category="Core" version="3.1.2">
 	<brief_description>
+		Internal mesh type.
 	</brief_description>
 	<description>
+		Mesh type used internally for collision calculations.
 	</description>
 	<tutorials>
 	</tutorials>