[SPARK-15502][DOC][ML][PYSPARK] add guide note that ALS only supports integer ids

This PR adds a note to clarify that the ML API for ALS only supports integers for user/item ids, and that other types for these columns can be used but the ids must fall within integer range.

(Refer [SPARK-14891](https://issues.apache.org/jira/browse/SPARK-14891)).

Also cleaned up a reference to `mllib` in the ML doc.

## How was this patch tested?
Built and viewed User Guide doc locally.

Author: Nick Pentreath <nickp@za.ibm.com>

Closes #13278 from MLnick/SPARK-15502-als-int-id-doc-note.