Document GLFW_CONTEXT_NO_ERROR caveat

docs/window.dox

@@ -302,6 +302,14 @@ The no error mode for OpenGL and OpenGL ES is described in detail by the
 [GL_KHR_no_error](https://www.opengl.org/registry/specs/KHR/no_error.txt)
 extension.
 
+@note This hint is experimental in its current state.  There are currently
+(October 2015) no corresponding WGL or GLX extensions.  That makes this hint
+a [hard constraint](@ref window_hints_hard) for those backends, as creation will
+fail if unsupported context flags are requested.  Once the extensions are
+available, they will be required and creation of `GL_KHR_no_error` contexts may
+fail on early drivers where this flag is supported without those extensions
+being listed.
+
 
 @subsubsection window_hints_values Supported and default values