docs: mention in the man page how M-V can insert any Unicode code point

This addresses https://savannah.gnu.org/bugs/?63055.

Inspired-by: Tasos Papastylianou <tpapastylianou@hotmail.com>

doc/nano.1

@@ -74,6 +74,12 @@ 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.
 .sp
+Any valid Unicode code point can be inserted into the buffer by typing
+\fBM\-V\fR followed by the hexadecimal digits of the code point (concluded
+with \fB<Space>\fR or \fB<Enter>\fR when it are fewer than six digits).
+A literal control code (except \fB^J\fR) can be inserted by typing
+\fBM\-V\fR followed by the pertinent keystroke.
+.sp
 The two lines at the bottom of the screen show some important commands;
 the built-in help (\fB^G\fR) lists all the available ones.
 The default key bindings can be changed via a \fInanorc\fR file -- see

doc/nano.texi

@@ -222,14 +222,18 @@ typed the key with that value.
 
 @item
 For any possible character, pressing @kbd{M-V} (Alt+V) and then typing a
-six-digit hexadecimal number (starting with @kbd{0} or @kbd{1}) will enter the
-corresponding Unicode character into the buffer.
+series of hexadecimal digits (at most six, or concluded with @kbd{Enter} or
+@kbd{Space}) will enter the corresponding Unicode character into the buffer.
 @end itemize
 
 For example, typing @kbd{Esc Esc 2 3 4} will enter the character "ê" ---
 useful when writing about a French party.  Typing @kbd{M-V 0 0 2 2 c 4}
 will enter the symbol "⋄", a little diamond.
 
+Typing @kbd{M-V} followed by anything other than a hexadecimal digit
+will enter this keystroke verbatim into the buffer, allowing the user
+to insert literal control codes (except @code{^J}) or escape sequences.
+
 @node Commands
 @section Commands
 
@@ -1619,7 +1623,8 @@ Switches to editing/viewing the previous buffer when multiple buffers are open.
 Switches to editing/viewing the next buffer when multiple buffers are open.
 
 @item verbatim
-Inserts the next keystroke verbatim into the file.
+Inserts the next keystroke verbatim into the file, or begins Unicode input
+when a hexadecimal digit is typed (@pxref{Entering Text} for details).
 
 @item tab
 Inserts a tab at the current cursor location.

doc/nanorc.5

@@ -828,7 +828,8 @@ Switches to editing/viewing the previous buffer when multiple buffers are open.
 Switches to editing/viewing the next buffer when multiple buffers are open.
 .TP
 .B verbatim
-Inserts the next keystroke verbatim into the file.
+Inserts the next keystroke verbatim into the file, or begins Unicode input
+when a hexadecimal digit is typed.
 .TP
 .B tab
 Inserts a tab at the current cursor location.