protocol: clarify wl_data_offer::selection

Clearly specify that switching focus within the same client
doesn't mean a new selection will be send.

Signed-off-by: Jan Grulich <jgrulich@redhat.com>
3 jobs for clarify-wl-offer in 1 minute and 10 seconds (queued for 11 seconds)
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passed #11881711
check-commit

00:00:15

 
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debian:buster@container-prep

00:00:32

 
  Build
passed #11881713
build-native

00:00:22