[JENKINS-73923] Add javadoc for SSH host key verification strategies

[MarkEWaite]

[JENKINS-73923] Add javadoc for SSH host key verification strategies

JENKINS-73923 notes that the git client plugin extensions page has no documentation. It says "This extension point has no Javadoc documentation".

Provide the documentation for the extension point and the four implementations of the extension point so that extension point documentation will include documentation after the next release of the git client plugin.

Much of the text is copied from the README or derived from the text in the README.

Testing done

Checked that the Javadoc renders as expected in Chrome.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue