Add to documentation of 'jump-to-register'

* lisp/register.el (jump-to-register):
* doc/emacs/regs.texi (Position Registers): Document that
jumping to a register can push the mark.

doc/emacs/regs.texi

@@ -80,7 +80,9 @@ information until you store something else in it.
 @kindex C-x r j
 @findex jump-to-register
   The command @kbd{C-x r j @var{r}} switches to the buffer recorded in
-register @var{r}, and moves point to the recorded position.  The
+register @var{r}, pushes a mark, and moves point to the recorded
+position.  (The mark is not pushed if point was already at the
+recorded position, or in successive calls to the command.)  The
 contents of the register are not changed, so you can jump to the saved
 position any number of times.
 

lisp/register.el

@@ -231,6 +231,7 @@ Interactively, reads the register using `register-read-with-preview'."
 (defalias 'register-to-point 'jump-to-register)
 (defun jump-to-register (register &optional delete)
   "Move point to location stored in a register.
+Push the mark if jumping moves point, unless called in succession.
 If the register contains a file name, find that file.
 \(To put a file name in a register, you must use `set-register'.)
 If the register contains a window configuration (one frame) or a frameset