docs: clarify background job exit codes #11906

Job	Run time
Testing chores	2m 23s
Acceptance test node versions	2s
Build artifacts	1m 25s
ubuntu-latest w/ Node.js 26.x (2/2)	5m 20s
windows-latest w/ Node.js 18.x (2/3)	9m 9s
ubuntu-latest w/ Node.js 18.x (1/2)	6m 45s
ubuntu-latest w/ Node.js 18.x (2/2)	5m 59s
windows-latest w/ Node.js 18.x (1/3)	6m 7s
ubuntu-latest w/ Node.js 20.x (1/2)	6m 3s
ubuntu-latest w/ Node.js 24.x (1/2)	6m 26s
ubuntu-latest w/ Node.js 20.x (2/2)	5m 39s
ubuntu-latest w/ Node.js 26.x (1/2)	6m 11s
macos-latest w/ Node.js 18.x (2/3)	4m 21s
ubuntu-latest w/ Node.js 22.x (2/2)	4m 43s
ubuntu-latest w/ Node.js 24.x (2/2)	5m 19s
macos-latest w/ Node.js 18.x (3/3)	3m 19s
ubuntu-latest w/ Node.js 22.x (1/2)	6m 25s
windows-latest w/ Node.js 22.x (3/3)	3m 58s
macos-latest w/ Node.js 22.x (1/3)	3m 18s
windows-latest w/ Node.js 22.x (2/3)	9m 26s
windows-latest w/ Node.js 18.x (3/3)	6m 12s
macos-latest w/ Node.js 22.x (2/3)	4m 44s
macos-latest w/ Node.js 18.x (1/3)	3m 27s
macos-latest w/ Node.js 22.x (3/3)	2m 23s
windows-latest w/ Node.js 22.x (1/3)	5m 10s
Big-endian debian w/ Node.js latest (${{ matrix.shard }})	0s
	2h 4m 14s

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