(fix) update documentation for cert/key usage

[bclodius]

PR Checklist:

• I have run npm test locally and all tests are passing.
• I have added/updated tests for any new behavior.
  
• If this is a significant change, an issue has already been created where the problem / solution was discussed: [N/A, or add link to issue here]
  

PR Description

Follow up to #1585 and #1289 for fixing documentation.

As new relic user their scripted api documentation brought me to this library as I was required to implement some two-way ssl monitors. Using the example with agentOptions I wasted some good time trying to get it to work until I found these issues.

I did not find usage for agentOptions useful except for protocol/ciphers specifications.

[stale]

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