While at it, also document the command to reset the shape to the default,
which we should probably use. See foot commit 49034bb7 (csi: let CSI 0 q mean
"switch to user configured cursor style", 2019-07-22).
As of today, the XTerm documentation is a not clear on this; in
XTerm itself, CSI 0 q may actually change the cursor because they have an
additional default cursor is configured differently..