doc: Document the unstability ack for Vala

3 jobs for unstable in 3 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #4126
librem5
build-debian-clang

00:02:16

passed #4125
librem5
build-debian-gcc

00:02:13

24.8%
passed #4127
librem5
build-debian-package

00:01:56