GL: make FramebufferClearColor searchable under GL keywords.

'cuz I was writing some raw GL for Emscripten tests and arrived at

    glClear(GL_COLOR);

without having any suspicion that this is just TOTALLY WRONG. Same
would go for Vulkan few years ahead when I'm so used to the Magnum
wrappers that I forget what the common pitfalls are.

src/Magnum/GL/AbstractFramebuffer.h

@@ -41,6 +41,7 @@ namespace Magnum { namespace GL {
 @brief Mask for framebuffer clearing
 
 @see @ref AbstractFramebuffer, @ref FramebufferClearMask
+@m_enum_values_as_keywords
 */
 enum class FramebufferClear: GLbitfield {
     Color = GL_COLOR_BUFFER_BIT,    /**< Color buffer */