Talk about flow control along with C-x C-s and C-s.

etc/TUTORIAL

@@ -441,6 +441,13 @@ work if the system should crash.
    This should print "Wrote .../TUTORIAL" at the bottom of the screen.
    On VMS it will print "Wrote ...[...]TUTORIAL."
 
+NOTE: On some systems, typing C-x C-s will freeze the screen and you
+will see no further output from Emacs.  This indicates that an
+operating system "feature" called "flow control" is intercepting the
+C-s and not letting it get through to Emacs.  To unfreeze the screen,
+type C-q.  Then see the section "Spontaneous Entry to Incremental
+Search" in the Emacs manual for advice on dealing with this "feature".
+
 To make a new file, just find it "as if" it already existed.  Then
 start typing in the text.  When you ask to "save" the file, Emacs
 will really create the file with the text that you have inserted.
@@ -665,6 +672,13 @@ to the next occurrence of 'cursor' just type C-s again.  If no such
 occurrence exists Emacs beeps and tells you that it is a failing
 search.  C-g would also terminate the search.
 
+NOTE: On some systems, typing C-s will freeze the screen and you will
+see no further output from Emacs.  This indicates that an operating
+system "feature" called "flow control" is intercepting the C-s and not
+letting it get through to Emacs.  To unfreeze the screen, type C-q.
+Then see the section "Spontaneous Entry to Incremental Search" in the
+Emacs manual for advice on dealing with this "feature".
+
 If you are in the middle of an incremental search and type <Rubout>,
 you'll notice that the last character in the search string is erased
 and the search backs up to the last place of the search.  For