[SPARK-13214][DOCS] update dynamicAllocation documentation
[SPARK-13214][DOCS] update dynamicAllocation documentation

Author: Bill Chambers <bill@databricks.com>

Closes #11094 from anabranch/dynamic-docs.

(cherry picked from commit 66e1383de2650a0f06929db8109a02e32c5eaf6b)
Signed-off-by: Andrew Or <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@databricks.com>

haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark,haowu80s/spark