docs(log): document `BaseHandler` #15595

Job	Run time
lint (ubuntu-latest)	23s
lint (windows-latest)	36s
test-node (latest, ubuntu-latest)	15s
test (v1.x, ubuntu-latest)	1m 3s
wasm (crypto/_wasm)	4s
test (v1.x, windows-latest)	1m 18s
publish-dry-run	12s
test (v1.x, macOS-latest)	57s
test (canary, ubuntu-latest)	1m 47s
test (canary, windows-latest)	2m 15s
test (canary, macOS-latest)	1m 37s
	10m 27s

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