docs: mention bindable function 'zero', for toggling the interface bars

This should have been part of commit a623f583 from six weeks ago.

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

doc/nano.texi

@@ -1715,6 +1715,9 @@ Toggles the presence of the two-line list of key bindings at the bottom of the s
 (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 zero
+Toggles the presence of title bar and status bar.
+
 @item constantshow
 Toggles the constant display of the current line, column, and character positions.
 

doc/nanorc.5

@@ -931,6 +931,9 @@ Toggles the presence of the two-line list of key bindings at the bottom of the s
 (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 zero
+Toggles the presence of title bar and status bar.
+.TP
 .B constantshow
 Toggles the constant display of the current line, column, and character positions.
 .TP