docs: Add about cowspace_size

7 jobs for config_cowspacesize in 20 minutes and 38 seconds (queued for 3 seconds)
Name Stage Failure
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_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