document behaviour when render target omitted (#8988)

docs/api/renderers/WebGLRenderer.html

@@ -263,10 +263,10 @@
 
 		<h3>[method:null setRenderTarget]( [page:WebGLRenderTarget renderTarget] )</h3>
 		<div>
-		renderTarget -- The [page:WebGLRenderTarget renderTarget] that needs to be activated.<br />
+		renderTarget -- The [page:WebGLRenderTarget renderTarget] that needs to be activated (optional).<br />
 		</div>
 		<div>
-		This method sets the active rendertarget.
+		This method sets the active rendertarget. If the parameter is omitted the canvas is set as the active rendertarget.
 		</div>
 
 		<h3>[method:boolean supportsCompressedTextureS3TC]()</h3>