Add instanceMatrix to shader documentation

This change documents some speed bumps I ran into while trying to write a shader for instanced objects. There are enough levels of indirection in reading ShaderChunks from source that figuring out how instanceMatrix is used there was too difficult for me.
Thankfully someone wrote [a medium article](https://medium.com/@pailhead011/instancing-with-three-js-36b4b62bc127) on the topic, but it took a lot of searching after bouncing off the official documentation.

As someone who has not done much work in shaders before, an explanation of what the instanceMatrix maps between and how it is typically consumed was also helpful, so I included that as well.

docs/api/en/renderers/webgl/WebGLProgram.html

@@ -92,6 +92,16 @@
 			attribute vec4 skinWeight;
 		#endif
 		</code>
+		<code>
+		#ifdef USE_INSTANCING
+			// Note that modelViewMatrix is not set when rendering an instanced model,
+			// but can be calculated from viewMatrix * modelMatrix.
+			//
+			// Basic Usage:
+			//   gl_Position = projectionMatrix * viewMatrix * modelMatrix * instanceMatrix * vec4(position, 1.0);
+			attribute mat4 instanceMatrix;
+		#endif
+		</code>
 		</div>
 
 		<h3>Fragment shader:</h3>