docs(log): document FileHandler #15869

Job	Run time
wasm (crypto/_wasm)	6s
lint (ubuntu-latest)	5s
lint (windows-latest)	16s
publish-dry-run	13s
test (v1.x, ubuntu-latest)	1m 20s
test-node (latest, ubuntu-latest)	2m 33s
test (v1.x, windows-latest)	1m 47s
test (v1.x, macOS-latest)	1m 16s
test (v2.x, ubuntu-latest)	2m 10s
test (v2.x, windows-latest)	2m 49s
test (v2.x, macOS-latest)	2m 10s
test (canary, ubuntu-latest)	2m 11s
test (canary, windows-latest)	3m 16s
test (canary, macOS-latest)	2m 17s
	22m 29s

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