[SPARK-12760][DOCS] inaccurate description for difference between local vs...

[SPARK-12760][DOCS] inaccurate description for difference between local vs cluster mode in closure handling

Clarify that modifying a driver local variable won't have the desired effect in cluster modes, and may or may not work as intended in local mode

Author: Sean Owen <sowen@cloudera.com>

Closes #10866 from srowen/SPARK-12760.