[SPARK-20484][MLLIB] Add documentation to ALS code

## What changes were proposed in this pull request?

This PR adds documentation to the ALS code.

## How was this patch tested?

Existing tests were used.

mengxr srowen

This contribution is my original work.  I have the license to work on this project under the Spark project’s open source license.

Author: Daniel Li <dan@danielyli.com>

Closes #17793 from danielyli/spark-20484.