Document rectangle-preview option more (Bug#27974)

* lisp/rect.el (string-rectangle):
* etc/NEWS.25: Mention `rectangle-preview'.

2 files changed, 6 insertions, 0 deletions

diff --git a/etc/NEWS.25 b/etc/NEWS.25
index 5201a300e01..aac60c46887 100644
--- a/etc/NEWS.25
+++ b/etc/NEWS.25
@@ -712,7 +712,9 @@ CLOS class and slot documentation.
 *** 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.
+Customize 'rectangle-preview' to nil for the old behavior.
 
 ** New font-lock functions 'font-lock-ensure' and 'font-lock-flush'.
 These should be used in preference to 'font-lock-fontify-buffer' when
diff --git a/lisp/rect.el b/lisp/rect.el
index 12e62b6a990..a62ed95b715 100644
--- a/lisp/rect.el
+++ b/lisp/rect.el
@@ -450,6 +450,10 @@ With a prefix (or a FILL) argument, also fill too short lines."
   "Replace rectangle contents with STRING on each line.
 The length of STRING need not be the same as the rectangle width.
 
+When called interactively and option `rectangle-preview' is
+non-nil, display the result as the user enters the string into
+the minibuffer.
+
 Called from a program, takes three args; START, END and STRING."
   (interactive
    (progn