Scripting documentation: Qt.rect's additional properties, TileMap.resize() #3479

(Whoops, did not mean to mark this one as a bug!) Two documentation things, and maybe a feature request:

1. The documentation for TileMap.resize() says

   This operation can currently only be applied to maps loaded from a file.

   However, it works just fine on open maps that are unsaved. I suspect this is left over from when maps always had to be saved to a file to be opened as documents in Tiled. I haven't thoroughly tested it so I don't know what wording to offer. Does it work for mapFormat.read()? If not, then perhaps appropriate wording would be "[...] maps open in Tiled as documents." If it does work for maps opened from file but not open as documents, then the wording could be "[...] maps open in Tiled as documents, or loaded from a file."

2. The documentation for Qt.rect only lists its x, y, width, and height properties, but it actually also has left, right, top, bottom properties. These appear to be read-only - trying to set them does not update them or change the rect's other properties. I think these should at least be documented as read-only, as they're very convenient! Better yet, if possible, they should be made settable, updating the rectangle's other properties as appropriate. QRect already has setter functions for them, e.g. https://doc.qt.io/qt-5/qrect.html#setBottom