fun/man: suppress spurious error msg

When doing something like 'man src/cmd/ksh93/sh.1', the 'unset -f'
in builtin_has_selfdoc() complained of an invalid function name.
Just suppress that so it is silently ignored.

src/cmd/ksh93/fun/man

@@ -59,7 +59,7 @@ namespace man
 
 	builtin_has_selfdoc()
 	{
-		[[ $(unset -f -- "$1"; whence -t -- "$1") == builtin ]] \
+		[[ $(unset -f -- "$1" 2>/dev/null; whence -t -- "$1") == builtin ]] \
 			&& [[ ! $1 =~ ^(:|true|false|echo)$ \
 			&& $'\n'$(builtin)$'\n' == *$'\n'"$(unset -f -- "$1"; whence -- "$1")"$'\n'* ]]
 	}