text-input: document behavior regarding multiple text-inputs

Currently protocol does not specify what should happen if multiple text-inputs are created by same client, which is why this is more or less undefined behavior currently in compositor implementations.

If client has created more than one text-input objects and surface owned by the client is focused, then compositor should send enter event to all text-input objects, in case of enable request however only one text-input should be enabled per client per seat.

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I am not sure what compositor should do in such cases? send protocol error? Simply ignore the enable requests? Force disable/send leave to previous text-input object?

Thoughts welcome!

CC: @dcz_purism