Allow menu items to be disabled by putting a - at the start of their

name, rather than just including #[dim] which still allowed them to be
chosen.

tmux.1

@@ -4674,6 +4674,8 @@ The name and command are formats, see the
 and
 .Sx STYLES
 sections.
+If the name begins with a hyphen (-), then the item is disabled (shown dim) and
+may not be chosen.
 The name may be empty for a separator line, in which case both the key and
 command should be omitted.
 .Pp