Get back clarification about public and private key usage

[BaurzhanSakhariev]

Hi there, thanks for the nice library! While trying to use it at CrateDB some minor doc issues have been noticed.

There was an issue referring to unclarity of private keys usage.

Also there was a general hint referred in some comments: 1, 2, 3 and a community post

All this was removed in https://github.com/auth0/java-jwt/commit/d0ebc3f50581f3341a9b214f32612926b8b1a201

This PR gets back those as it's still might be unclear.

Checklist

• [x] I have read the Auth0 general contribution guidelines
• [x] I have read the Auth0 Code of Conduct
• [x] All existing and new tests complete without errors