docs: Adds `generateMipMaps` option to WebGLRenderTarget

docs/api/en/renderers/WebGLRenderTarget.html

@@ -35,6 +35,7 @@
 		[page:Constant wrapT] - default is [page:Textures ClampToEdgeWrapping]. <br />
 		[page:Constant magFilter] - default is [page:Textures LinearFilter]. <br />
 		[page:Constant minFilter] - default is [page:Textures LinearFilter]. <br />
+                [page:Boolean generateMipmaps] - default is *false*. Set this to true if you need it.<br /><br />
 		[page:Constant format] - default is [page:Textures RGBAFormat]. <br />
 		[page:Constant type] - default is [page:Textures UnsignedByteType]. <br />
 		[page:Number anisotropy] - default is *1*. See [page:Texture.anisotropy]<br />