doc: add and unify return statements in crypto.md
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Conform return statements to the style guide and tool parsers.
Also bring back a description fragment that seems to be erroneously deleted in https://github.com/nodejs/node/commit/1e07acd476309e7ddc4981160b89731b61a31179 (cc @tniessen)