document that Mesos cluster mode supports python
document that Mesos cluster mode supports python

update docs to be consistent with SPARK-14645 https://issues.apache.org/jira/browse/SPARK-14645

Author: Michael Gummelt <mgummelt@mesosphere.io>

Closes #14514 from mgummelt/fix-docs.

(cherry picked from commit 7aaa5a01c1ee142663d28e98eb501fffc5a5cc46)
Signed-off-by: Sean Owen <291c18f3fb7528c712d9098b0e50a515ea0b91d5@cloudera.com>

gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark,gioenn/xSpark