doc: explain `hex` encoding in Buffer API
fixes: https://github.com/nodejs/node/issues/29786 refs: https://github.com/nodejs/node/pull/29792 refs: https://github.com/nodejs/node/issues/24491
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines