Document easy ways of typing undo key on TTY frames

* doc/emacs/basic.texi (Basic Undo): Document the easiest way to
type the undo key.

* etc/tutorials/TUTORIAL: Describe typing C-_ without the Shift
key.

doc/emacs/basic.texi

@@ -461,6 +461,14 @@ Normally, this command undoes the last change, moving point back to
 where it was before the change.  The undo command applies only to
 changes in the buffer; you can't use it to undo cursor motion.
 
+  On a graphics terminal (including text-mode frames displayed by a
+terminal emulator, such as @command{xterm}), the easiest way to invoke
+@code{undo} is with @kbd{C-/}; that doesn't need the Shift key.  On a
+text terminal, @kbd{C-/} does not exist, but in many cases you can type
+@kbd{C-_} without the Shift key (in effect pressing @kbd{C--}) and it
+will work anyway, at least with keyboards that produce the US ASCII
+character set.
+
   Although each editing command usually makes a separate entry in the
 undo records, very simple commands may be grouped together.
 Sometimes, an entry may cover just part of a complex command.

etc/tutorials/TUTORIAL

@@ -473,6 +473,7 @@ to undo insertion of text.)
 >> Kill this line with C-k, then type C-/ and it should reappear.
 
 C-_ is an alternative undo command; it works exactly the same as C-/.
+On some text terminals, you can omit the shift key when you type C-_.
 On some text terminals, typing C-/ actually sends C-_ to Emacs.
 Alternatively, C-x u also works exactly like C-/, but is a little less
 convenient to type.