[SPARK-15500][DOC][ML][PYSPARK] Remove default value in Param doc field in ALS

Remove "Default: MEMORY_AND_DISK" from `Param` doc field in ALS storage level params. This fixes up the output of `explainParam(s)` so that default values are not displayed twice.

We can revisit in the case that [SPARK-15130](https://issues.apache.org/jira/browse/SPARK-15130) moves ahead with adding defaults in some way to PySpark param doc fields.

Tests N/A.

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

Closes #13277 from MLnick/SPARK-15500-als-remove-default-storage-param.