* Don't treat render targets like textures.

Expect Texture instances from now on. Added backwards compatibility
measures to ensure that render targets can still act as textures for the
time being. A deprecation warning will be given. Resolves #8615.

* Added comments to clear up RT-Cube detection.

* Revised the previously added comments.

* Added a TODO note.

Forgot to include this one in the last commit.

* Documentation: updated WebGLRenderTarget.

* Docs: added more Texture info to WebGLRenderTarget.

Also removed an unnecessary html line break.

* Docs: added a notification to ShaderMaterial.

WebGLRenderTarget and WebGLRenderTargetCube must not be used as
uniforms. Their texture instance must be used instead.

* Docs: revised a notification in ShaderMaterial.