[SPARK-20011][ML][DOCS] Clarify documentation for ALS 'rank' parameter

## What changes were proposed in this pull request?

API documentation and collaborative filtering documentation page changes to clarify inconsistent description of ALS rank parameter.

 - [DOCS] was previously: "rank is the number of latent factors in the model."
 - [API] was previously:  "rank - number of features to use"

This change describes rank in both places consistently as:

 - "Number of features to use (also referred to as the number of latent factors)"

Author: Chris Snow <chris.snowuk.ibm.com>

Author: christopher snow <chsnow123@gmail.com>

Closes #17345 from snowch/SPARK-20011.