Backends: standardized top of file comments.

backends/imgui_impl_allegro5.cpp

backends/imgui_impl_allegro5.h

backends/imgui_impl_android.cpp

backends/imgui_impl_android.h

backends/imgui_impl_dx10.cpp

backends/imgui_impl_dx10.h

backends/imgui_impl_dx11.cpp

backends/imgui_impl_dx11.h

backends/imgui_impl_dx12.cpp

backends/imgui_impl_dx12.h

backends/imgui_impl_dx9.cpp

backends/imgui_impl_dx9.h

backends/imgui_impl_glfw.cpp

backends/imgui_impl_glfw.h

backends/imgui_impl_glut.cpp

backends/imgui_impl_glut.h

backends/imgui_impl_metal.h

backends/imgui_impl_metal.mm

backends/imgui_impl_opengl2.cpp

backends/imgui_impl_opengl2.h