[SPARK-4884]: Improve Partition docs

Rewording was based on this discussion: http://apache-spark-developers-list.1001551.n3.nabble.com/RDD-data-flow-td9804.html
This is the associated JIRA ticket: https://issues.apache.org/jira/browse/SPARK-4884

Author: Madhu Siddalingaiah <madhu@madhu.com>

Closes #3722 from msiddalingaiah/master and squashes the following commits:

79e679f [Madhu Siddalingaiah] [DOC]: improve documentation
51d14b9 [Madhu Siddalingaiah] Merge remote-tracking branch 'upstream/master'
38faca4 [Madhu Siddalingaiah] Merge remote-tracking branch 'upstream/master'
cbccbfe [Madhu Siddalingaiah] Documentation: replace <b> with <code> (again)
332f7a2 [Madhu Siddalingaiah] Documentation: replace <b> with <code>
cd2b05a [Madhu Siddalingaiah] Merge remote-tracking branch 'upstream/master'
0fc12d7 [Madhu Siddalingaiah] Documentation: add description for repartitionAndSortWithinPartitions

(cherry picked from commit d5a596d4188bfa85ff49ee85039f54255c19a4de)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>
SPARK-3428. TaskMetrics for running tasks is missing GC time metrics

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3684 from sryza/sandy-spark-3428 and squashes the following commits:

cb827fe [Sandy Ryza] SPARK-3428. TaskMetrics for running tasks is missing GC time metrics

(cherry picked from commit 283263ffaa941e7e9ba147cf0ad377d9202d3761)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>