Enhance window with OpenGL with viewport and documentation

[damusss]

edit: if the title wasn't clear, I'm referring to pygame.window.Window, not pygame.display
Pull request #2659 was finally merged so it won't appear on the pull requests list and some important comments might get lost, but I think they need to be addressed before 2.5.0 official release.
Once they are, this issue can be closed.

• Update docs of Window.flip() with latest changes (by MyreMylar)
• Resize the OpenGL viewport on the event watch when the size changes (for full details check Window(opengl=True) will now create an OpenGL context #2659 (comment) )

About the second one, a contributor also said they had even more enhancements in mind for the event watch, and this can be the right place to put them in.