docs(log): document FileHandler #15867

Job	Run time
test-node (latest, ubuntu-latest)	14s
wasm (crypto/_wasm)	5s
lint (ubuntu-latest)	8s
test (v1.x, ubuntu-latest)	17s
lint (windows-latest)	17s
test (v1.x, windows-latest)	29s
test (v1.x, macOS-latest)	19s
publish-dry-run	15s
test (v2.x, ubuntu-latest)	2m 3s
test (v2.x, windows-latest)	2m 22s
test (v2.x, macOS-latest)	1m 58s
test (canary, ubuntu-latest)	1m 59s
test (canary, windows-latest)	2m 27s
test (canary, macOS-latest)	2m 22s
	15m 15s

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