process: add _getActiveHandles documentation

[yashLadha]

Documenting activeHandles documentation in the process API which can be used by the developers to make functions/modules to track the health of their node process.

Though i am aware of the reason that why it is not documented yet, even though being very much helpful for instrumentation of process stats. I might be lacking some information around the context but wanted to float this PR so that iteration can be done on this PR and developers are aware of this awesome function.

[RaisinTen]

PR to make it public: https://github.com/nodejs/node/pull/37262