[osx] Document isatty().

osx.c

@@ -802,6 +802,13 @@ static int os_fadvise(lua_State *L)
     return os_result(L, ec, NULL);
 }
 
+/**
+ * Test whether an opened file handle references a character device (TTY).
+ *
+ * @tparam File f an opened file handle
+ * @treturn boolean true if file references a character device, false otherwise.
+ * @function isatty
+ */
 static int os_isatty(lua_State *L)
 {
     luaL_Stream *stream = luaL_checkudata(L, 1, LUA_FILEHANDLE);