Improve rectangle-mark-mode documentation regarding mark activation

Improve rectangle-mark-mode documentation regarding mark activation

* doc/emacs/killing.texi: Describe how rectangle-mark-mode works when
Transient Mark mode is off.  Qualify discussion of
rectangle-exchange-point-and-mark to say that it cycles the four
corners only when the region is active.
* lisp/rect.el (rectangle-mark-mode): State that rectangle-mark-mode
does not activate the mark when Transient Mark mode is off.  Reword
sentence about how long the mode lasts to account for when Transient
Mark mode is off.  (Bug#42663)