Make it explicit that `read` cannot be used with keymap file descriptors.

This is true at least for compositors using memfds where multiple clients might receive the same file description. I'm not aware of any clients trying to do this but it seems possible that someone might assume that this works.

Something like

It is not guaranteed that this file descriptor can be used for any operations other than mmap(2). Clients should not use read(2) to read the keymap.

might be useful.