Explicitly document the effect of binding globals etc. multiple times

From #426 (comment 2257141) by @ehopperdietzel:

Returning to the main topic, I believe it would be beneficial to explicitly state in the protocols documentation that clients are permitted to bind multiple times to these globals or create multiple instances of wl_pointer, wl_keyboard, wl_touch from the same wl_seat, and clarify that the compositor must send events to each one of them individually and so on like here. Personally, I initially considered it a client bug to do so, assuming it was so obvious that it didn't even need mentioning in the documentation. However, I was mistaken. Perhaps others could make the same mistake.