Clearer explanation of back buffer regions

**Your Godot version:**
3.3.stable.official

**Issue description:**
`BackBufferCopy` in rect mode and `Visualserver.canvas_item_set_copy_to_backbuffer` both use global screen-space coordinates/dimensions, which is an important detail omitted from the documentation of both. (A `BackBufferCopy` rect does not inherit parent transforms and must be transformed manually. Also, screen-space is y-flipped compared to global world-space and must be transformed manually when using either)

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_backbuffercopy.html
https://docs.godotengine.org/en/stable/classes/class_visualserver.html#class-visualserver-method-canvas-item-set-copy-to-backbuffer