[SPARK-13992][CORE][PYSPARK][FOLLOWUP] Update OFF_HEAP semantics for Java api and Python api

## What changes were proposed in this pull request?

- updated `OFF_HEAP` semantics for `StorageLevels.java`
- updated `OFF_HEAP` semantics for `storagelevel.py`

## How was this patch tested?

no need to test

Author: Liwei Lin <lwlin7@gmail.com>

Closes #12126 from lw-lin/storagelevel.py.