doc: add TLSSocket.isSessionReused() docs
The API has existed forever and is used in our unit tests. It is supported for TLS1.3 as well as 1.2 and useful for troubleshooting, so it should be documented.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines