Proofread and improve the whole class reference

- Document a few more properties and methods
- Add more information to many classes
- Fix lots of typos and gramar mistakes
- Use [code] tags for parameters consistently
- Use [b] and [i] tags consistently
- Put "Warning:" and "Note:" on their own line to be more visible,
  and make them always bold
- Tweak formatting in code examples to be more readable
- Use double quotes consistently
- Add more links to third-party technologies

doc/classes/MeshTexture.xml

@@ -12,13 +12,13 @@
 	</methods>
 	<members>
 		<member name="base_texture" type="Texture" setter="set_base_texture" getter="get_base_texture">
-			Set the base texture that the Mesh will use to draw.
+			Sets the base texture that the Mesh will use to draw.
 		</member>
 		<member name="image_size" type="Vector2" setter="set_image_size" getter="get_image_size">
-			Set the size of the image, needed for reference.
+			Sets the size of the image, needed for reference.
 		</member>
 		<member name="mesh" type="Mesh" setter="set_mesh" getter="get_mesh">
-			Set the mesh used to draw. It must be a mesh using 2D vertices.
+			Sets the mesh used to draw. It must be a mesh using 2D vertices.
 		</member>
 	</members>
 	<constants>