fix(fs): improve the docs and error message of `ensureSymlink(Sync)` #15929

Job	Run time
test-node (latest, ubuntu-latest)	16s
test (v1.x, ubuntu-latest)	1m 17s
lint (ubuntu-latest)	26s
test (v1.x, windows-latest)	2m 18s
lint (windows-latest)	38s
wasm (crypto/_wasm)	8s
test (v1.x, macOS-latest)	1m 26s
test (v2.x, ubuntu-latest)	2m 9s
test (v2.x, windows-latest)	2m 54s
test (v2.x, macOS-latest)	3m 14s
test (canary, ubuntu-latest)	2m 34s
test (canary, windows-latest)	2m 44s
test (canary, macOS-latest)	2m 12s
	22m 16s

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