Add documentation for `--include-extended-types` and `--help` flags.

[theMomax]

(Bug/issue #, if applicable: apple/swift-docc#210)

Summary

Adds a new doc page for the --include-extended-types flag.

This PR also adds a hint on how to access the --help flag to the landing page. I think that many people don't know you have to add the swift package plugin subcommand to get access to the plugin's help text.

Dependencies

None.

Testing

N/A

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [x] Added testsN/A
• [x] Ran the ./bin/test script and it succeeded
• [x] Updated documentation if necessary

[theMomax]

@swift-ci please test

[theMomax]

@swift-ci please test