[SPARK-7533] [YARN] Decrease spacing between AM-RM heartbeats.

Added faster RM-heartbeats on pending container allocations with multiplicative back-off.
Also updated related documentations.

Author: ehnalis <zoltan.zvara@gmail.com>

Closes #6082 from ehnalis/yarn and squashes the following commits:

a1d2101 [ehnalis] MIss-spell fixed.
90f8ba4 [ehnalis] Changed default HB values.
6120295 [ehnalis] Removed the bug, when allocation heartbeat would not start from initial value.
08bac63 [ehnalis] Refined style, grammar, removed duplicated code.
073d283 [ehnalis] [SPARK-7533] [YARN] Decrease spacing between AM-RM heartbeats.
d4408c9 [ehnalis] [SPARK-7533] [YARN] Decrease spacing between AM-RM heartbeats.