audio-channels: Add rear-left/right-of-center channels

In the process, clarify what the channel positions mean in the context
ofr 5.1/7.1 configurations.
6 jobs for audio-channels in 11 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Preparation
failed #57005
gst indent

00:00:11

failed #57006
manifest

00:00:09

failed #56456
gst indent

00:00:09

failed #56455
manifest

00:00:09

 
  Build
skipped #56457
build fedora x86_64
 
  Full Builds
skipped #56458
build android arm64 api28
 
Name Stage Failure
failed
gst indent Preparation
Pulling docker image registry.freedesktop.org/gstreamer/gst-ci/amd64/gst-indent:6f7e01e1e30a73efa880acdc8e911f1f20c58dbb ...
Using docker image sha256:fea02534ab1a96a72e447d47ee0762cf71bf59928ff027b7dd7fb4fabd418f40 for registry.freedesktop.org/gstreamer/gst-ci/amd64/gst-indent:6f7e01e1e30a73efa880acdc8e911f1f20c58dbb ...
Running on runner-81280d8f-project-1791-concurrent-0 via fdo-gitlab-gce-runner3...
Cloning repository...
Cloning into '/builds/arun/gst-plugins-base'...
fatal: write error: No space left on device
fatal: index-pack failed
/bin/bash: line 69: cd: /builds/arun/gst-plugins-base: No such file or directory
ERROR: Job failed: exit code 1
failed
manifest Preparation There has been a runner system failure, please try again
  on fdo-gitlab-gce-runner3 81280d8f
Using Docker executor with image registry.freedesktop.org/gstreamer/gst-ci/amd64/build-manifest:f085d34a32cb8cac95cbd420344efeaa647d08d5 ...
Pulling docker image registry.freedesktop.org/gstreamer/gst-ci/amd64/build-manifest:f085d34a32cb8cac95cbd420344efeaa647d08d5 ...
Using docker image sha256:9bf8b9996ca962bd6fee0901e95ace4872017378721dfd737d5a5554be9b673b for registry.freedesktop.org/gstreamer/gst-ci/amd64/build-manifest:f085d34a32cb8cac95cbd420344efeaa647d08d5 ...
Running on runner-81280d8f-project-1791-concurrent-1 via fdo-gitlab-gce-runner3...
Skipping Git repository setup
Skipping Git checkout
Skipping Git submodules setup
ERROR: Job failed (system failure): Error response from daemon: mkdir /var/lib/docker/overlay2/79c66435cef829142895ab2267b4eca2d5e0778dbb73544f7ba50175f73c901a/work: no space left on device (executor_docker.go:921:0s)