Document the "pseudo-fullscreen" hotkey

[Screwtapello]

At least in the GTK+2 build, there are hotkeys for "Toggle Fullscreen" and "Toggle Pseudo-Fullscreen". We should figure out how those are different and document it somewhere.

[TriggeredSnake]

Psuedo Fullscreen on my version (might not be GTK+2) activates borderless windowed fullscreen as long as the status bar is enabled. Disable that and it just works the same as normal fullscreen.