doc: updated tls docs for tls socket subjectaltname
The documentation for tlsSocket.getPeerCertificate() stated that the certificate object returned subjectaltname as an array however it actually returned a concatenated list of strings as a single string docs have been updated to reflect this
Fixes: https://github.com/nodejs/node/issues/27721
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines