bpo-1529353: Explain Shell text squeezing in the IDLE doc. (GH-10169)

(cherry picked from commit 68d6dc0770288075504635a8e42696070823fd69)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
diff --git a/Doc/library/idle.rst b/Doc/library/idle.rst
index 0eb1b44..aaf4e9e 100644
--- a/Doc/library/idle.rst
+++ b/Doc/library/idle.rst
@@ -341,11 +341,18 @@
 Clear Breakpoint
    Clear the breakpoint on that line.
 
-Shell and Output windows have the following.
+Shell and Output windows also have the following.
 
 Go to file/line
    Same as in Debug menu.
 
+The Shell window also has an output squeezing facility explained in the
+the *Python Shell window* subsection below.
+
+Squeeze
+   If the cursor is over an output line, squeeze all the output between
+   the code above and the prompt below down to a 'Squeezed text' label.
+
 
 Editing and navigation
 ----------------------
@@ -477,6 +484,9 @@
 Python Shell window
 ^^^^^^^^^^^^^^^^^^^
 
+The editing features described above work when entering code interactively.
+IDLE's Shell window also responds to the following keys.
+
 * :kbd:`C-c` interrupts executing command
 
 * :kbd:`C-d` sends end-of-file; closes window if typed at a ``>>>`` prompt
@@ -492,6 +502,15 @@
 
   * :kbd:`Return` while on any previous command retrieves that command
 
+Shell has a special facility for squeezing output lines down to a
+'Squeezed text' label.  This is done automatically for output over N lines
+(N = 50 by default).  N can be changed in the PyShell section of the General
+page of the Settings dialog.  Output with fewer lines can be squeezed by
+right clicking on the output.  This can be useful for extra long lines.
+
+Squeezed output is expanded in place by double-clicking the label.
+It can also be sent to the clipboard or a separate view window by
+right-clicking the label.
 
 Text colors
 ^^^^^^^^^^^