configs/*/airootfs/etc/systemd/network/20-ethernet.network: add a comment to...

configs/*/airootfs/etc/systemd/network/20-ethernet.network: add a comment to document why an interface name glob is used

This documents the changes made in !177 inside the .network files themselves.

Related to #142.
7 jobs for document-interface-glob-in-network-files in 8 minutes and 11 seconds (queued for 13 seconds)
Name Stage Failure
failed
build_long: [baseline netboot] Build
Running after script...
$ cp -v -- output/metrics.txt .
cp: cannot stat 'output/metrics.txt': No such file or directory
Uploading artifacts for failed job
Uploading artifacts...
WARNING: metrics.txt: no matching files
ERROR: No files to upload
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
build_long: [releng netboot] Build
Running after script...
$ cp -v -- output/metrics.txt .
cp: cannot stat 'output/metrics.txt': No such file or directory
Uploading artifacts for failed job
Uploading artifacts...
WARNING: metrics.txt: no matching files
ERROR: No files to upload
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
build_short: [baseline bootstrap] Build
Running after script...
$ cp -v -- output/metrics.txt .
cp: cannot stat 'output/metrics.txt': No such file or directory
Uploading artifacts for failed job
Uploading artifacts...
WARNING: metrics.txt: no matching files
ERROR: No files to upload
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
build_short: [releng bootstrap] Build
Running after script...
$ cp -v -- output/metrics.txt .
cp: cannot stat 'output/metrics.txt': No such file or directory
Uploading artifacts for failed job
Uploading artifacts...
WARNING: metrics.txt: no matching files
ERROR: No files to upload
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
build_long: [releng iso] Build
Running after script...
$ cp -v -- output/metrics.txt .
cp: cannot stat 'output/metrics.txt': No such file or directory
Uploading artifacts for failed job
Uploading artifacts...
WARNING: metrics.txt: no matching files
ERROR: No files to upload
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1

failed
build_long: [baseline iso] Build
Running after script...
$ cp -v -- output/metrics.txt .
cp: cannot stat 'output/metrics.txt': No such file or directory
Uploading artifacts for failed job
Uploading artifacts...
WARNING: metrics.txt: no matching files
ERROR: No files to upload
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1