Improve ML attribute documentation

ML attribute documentation is currently minimal. This has led to confusion in some Spark PRs about how to use them.

We should add:

• Scala doc
• examples in the programming guide

The docs should make at least these items clear:

• What the different attribute types are
• How an attribute and attribute group differ
• Example usage creating, modifying, and reading attributes
• Explanation that missing attributes are OK and can be computed/added lazily