update docs for setRenderTarget (#23851)

docs/api/en/renderers/WebGLRenderer.html

@@ -429,7 +429,7 @@
 		<h3>[method:undefined setRenderTarget]( [param:WebGLRenderTarget renderTarget], [param:Integer activeCubeFace], [param:Integer activeMipmapLevel] )</h3>
 		<p>
 		renderTarget -- The [page:WebGLRenderTarget renderTarget] that needs to be activated. When *null* is given, the canvas is set as the active render target instead.<br />
-		activeCubeFace -- Specifies the active cube side (PX 0, NX 1, PY 2, NY 3, PZ 4, NZ 5) of [page:WebGLCubeRenderTarget] (optional).<br />
+		activeCubeFace -- Specifies the active cube side (PX 0, NX 1, PY 2, NY 3, PZ 4, NZ 5) of [page:WebGLCubeRenderTarget]. When passing a [page:WebGLArrayRenderTarget] or [page:WebGL3DRenderTarget] this indicates the z layer to render in to (optional).<br />
 		activeMipmapLevel -- Specifies the active mipmap level (optional).<br /><br />
 		This method sets the active rendertarget.
 		</p>

docs/api/zh/renderers/WebGLRenderer.html

@@ -368,7 +368,7 @@
 		<h3>[method:undefined setRenderTarget]( [param:WebGLRenderTarget renderTarget], [param:Integer activeCubeFace], [param:Integer activeMipmapLevel] )</h3>
 		<p>
 		renderTarget -- 需要被激活的[page:WebGLRenderTarget renderTarget](可选)。若此参数为空，则将canvas设置成活跃render target。<br />
-		activeCubeFace -- Specifies the active cube side (PX 0, NX 1, PY 2, NY 3, PZ 4, NZ 5) of [page:WebGLCubeRenderTarget] (optional).<br />
+		activeCubeFace -- Specifies the active cube side (PX 0, NX 1, PY 2, NY 3, PZ 4, NZ 5) of [page:WebGLCubeRenderTarget]. When passing a [page:WebGLArrayRenderTarget] or [page:WebGL3DRenderTarget] this indicates the z layer to render in to (optional).<br />
 		activeMipmapLevel -- Specifies the active mipmap level (optional).<br /><br />
 		该方法设置活跃rendertarget。
 		</p>