docs: mention that also Ctrl increases the stride when selecting text

The Alt key together with Shift is a special workaround for when
Shift+PgUp and Shift+PgDn don't work on the user's terminal.  But
probably it is more common to use the Ctrl key together with Shift
to select larger pieces of text.  So mention this possibility.

doc/nano.1

@@ -55,7 +55,8 @@ cutbuffer.  One can also save the marked text to a file with \fB^O\fR, or
 spell check it with \fB^T\fR.
 .PP
 On some terminals, text can be selected also by holding down Shift while
-using the arrow keys.  Holding down the Alt key too will increase the stride.
+using the arrow keys.  Holding down the Ctrl or Alt key too will increase
+the stride.
 Any cursor movement without Shift being held will cancel such a selection.
 .PP
 The two lines at the bottom of the screen show some important commands;

doc/nano.texi

@@ -506,8 +506,9 @@ This selection can now be cut or copied in its entirety with a single
 @kbd{^K} or @kbd{M-6}.  Or the selection can be used to limit the scope of
 a search-and-replace (@kbd{^\}) or spell-checking session (@kbd{^T}).
 
-On some terminals, it is also possible to select text by holding down
-@kbd{Shift} together with the cursor keys.  Such a selection is cancelled
+On some terminals, text can be selected also by holding down @kbd{Shift}
+while using the cursor keys.  Holding down the @kbd{Ctrl} or @kbd{Alt}
+key too will increase the stride.  Such a selection is cancelled
 upon any cursor movement where @kbd{Shift} isn't held.
 
 Cutting or copying selected text will toggle the mark off automatically.