docs: mention M-Z (for toggling the interface) among the Feature Toggles

And rephrase the M-Y description a bit, for shortness and variety.

doc/nano.texi

@@ -761,10 +761,13 @@ for more details).  The following global toggles are available:
 @item Expert/No Help
 @kbd{M-X} toggles the @option{-x} (@option{--nohelp}) command-line option.
 
-@item Color Syntax Highlighting
+@item Syntax Coloring
-@kbd{M-Y} toggles color syntax highlighting, when your nanorc defines syntaxes
+@kbd{M-Y} toggles syntax coloring, when your nanorc defines syntaxes
 (@pxref{Syntax Highlighting}).
 
+@item Hidden Interface
+@kbd{M-Z} toggles the @option{-0} (@option{--zero}) command-line option.
+
 @end table