docs: mention that M-X toggle is special, because available in most menus

doc/nano.1

@@ -331,6 +331,10 @@ hard-wrapping of long lines, \fBM\-S\fR toggles soft-wrapping,
 \fBM\-N\fR toggles line numbers, \fBM\-M\fR toggles the mouse,
 \fBM\-I\fR auto-indentation, and \fBM\-X\fR the help lines.
 See at the end of the \fB^G\fR help text for a complete list.
+.sp
+The \fBM\-X\fR toggle is special: it works in all menus except
+the help viewer and the linter.  All other toggles work in
+the main menu only.
 
 .SH FILES
 When \fB\-\-rcfile\fR is given, \fBnano\fR will read just the specified file

doc/nano.texi

@@ -1528,6 +1528,8 @@ Goes to the last file when using the file browser (reading or writing files).
 
 @item nohelp
 Toggles the presence of the two-line list of key bindings at the bottom of the screen.
+(This toggle is special: it is available in all menus except the help viewer
+and the linter.  All further toggles are available in the main menu only.)
 
 @item constantshow
 Toggles the constant display of the current line, column, and character positions.

doc/nanorc.5

@@ -837,6 +837,8 @@ Goes to the last file when using the file browser (reading or writing files).
 .TP
 .B nohelp
 Toggles the presence of the two-line list of key bindings at the bottom of the screen.
+(This toggle is special: it is available in all menus except the help viewer
+and the linter.  All further toggles are available in the main menu only.)
 .TP
 .B constantshow
 Toggles the constant display of the current line, column, and character positions.