[SPARK-6469] Improving documentation on YARN local directories usage

Clarify the local directories usage in YARN

Author: Christophe Préaud <christophe.preaud@kelkoo.com>

Closes #5165 from preaudc/yarn-doc-local-dirs and squashes the following commits:

6912b90 [Christophe Préaud] Fix some formatting issues.
4fa8ec2 [Christophe Préaud] Merge remote-tracking branch 'upstream/master' into yarn-doc-local-dirs
eaaf519 [Christophe Préaud] Clarify the local directories usage in YARN
436fb7d [Christophe Préaud] Revert "Clarify the local directories usage in YARN"
876ae5e [Christophe Préaud] Clarify the local directories usage in YARN
608dbfa [Christophe Préaud] Merge remote-tracking branch 'upstream/master'
a49a2ce [Christophe Préaud] Merge remote-tracking branch 'upstream/master'
9ba89ca [Christophe Préaud] Ensure that files are fetched atomically
54419ae [Christophe Préaud] Merge remote-tracking branch 'upstream/master'
c6a5590 [Christophe Préaud] Revert commit 8ea871f8130b2490f1bad7374a819bf56f0ccbbd
7456a33 [Christophe Préaud] Merge remote-tracking branch 'upstream/master'
8ea871f [Christophe Préaud] Ensure that files are fetched atomically