Add note that multiple lights loop commands (forwardlights / lightvolumes) are not supported in the renderpath. Closes #1671.

Docs/Reference.dox

@@ -1514,6 +1514,8 @@ The available commands are:
 
 Scenepass, quad, forwardlights and lightvolumes commands all allow command-global shader compilation defines, shader parameters and textures to be defined. For example in deferred rendering, the lightvolumes command would bind the G-buffer textures to be able to calculate the lighting. Note that when binding command-global textures, these are (for optimization) bound only once in the beginning of the command. If the texture binding is overwritten by an object's material, it is "lost" until the end of the command. Therefore the command-global textures should be in units that are not used by materials.
 
+Note that it's legal for only one forwardlights or one lightvolumes command to exist in the renderpath.
+
 A render path can be loaded from a main XML file by calling \ref RenderPath::Load "Load()", after which other XML files (for example one for each post-processing effect) can be appended to it by calling \ref RenderPath::Append "Append()". Rendertargets and commands can be enabled or disabled by calling \ref RenderPath::SetEnabled "SetEnabled()" to switch eg. a post-processing effect on or off. To aid in this, both can be identified by tag names, for example the bloom effect uses the tag "Bloom" for all of its rendertargets and commands.
 
 It is legal to both write to the destination viewport and sample from it during the same command: pingpong copies of its contents will be made automatically. If the viewport has hardware multisampling on, the multisampled backbuffer will be resolved to a texture before sampling it.