xdg-shell: explain how clients need to perform an initial commit

This wasn't explicit reading the mapping requirements.

Signed-off-by: Simon Ser <contact@emersion.fr>
2 jobs for master in 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Containers Build
passed #2621546
container_build

00:00:17

 
  Test
passed #2621547
test

00:00:30