[v9.x backport] doc: fix a typo in util.isDeepStrictEqual & doc: add missing `Returns` in fs & util
This is a backport PR related to https://github.com/nodejs/node/pull/18775 .
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc