Document new features of Rectangle mode

* doc/emacs/killing.texi (Rectangles): Document "C-x C-x" in
rectangle-mark-mode.

doc/emacs/killing.texi

@@ -853,6 +853,19 @@ so in a rectangular fashion, and killing and yanking operate on the
 rectangle.  @xref{Killing}.  The mode persists only as long as the
 region is active.
 
+Unlike the standard region, the region-rectangle can have its corners
+extended past the end of buffer, or inside stretches of white space
+that point normally cannot enter, like the TAB.
+
+@findex rectangle-exchange-point-and-mark
+@findex exchange-point-and-mark@r{, in rectangle-mark-mode}
+@kindex C-x C-x@r{, in rectangle-mark-mode}
+When the region is in rectangle-mark-mode, @kbd{C-x C-x} runs the
+command @code{rectangle-exchange-point-and-mark}, which cycles between
+the four corners of the region-rectangle.  This comes in handy if you
+want to modify the dimensions of the region-rectangle before invoking
+an operation on the marked text.
+
 @node CUA Bindings
 @section CUA Bindings
 @findex cua-mode

etc/NEWS

@@ -634,7 +634,11 @@ respectively, `show-paren-when-point-inside-paren' or
 *** Strings after `:documentation' are highlighted as docstrings.
 
 ** Rectangle editing
+
++++
 *** Rectangle Mark mode can have corners past EOL or in the middle of a TAB.
+
++++
 *** C-x C-x in rectangle-mark-mode now cycles through the four corners.
 *** `string-rectangle' provides on-the-fly preview of the result.