[SPARK-3240] Adding known issue for MESOS-1688

When using Mesos with the fine-grained mode, a Spark job can run into a dead lock on low allocatable memory on Mesos slaves. As a work-around 32 MB (= Mesos MIN_MEM) are allocated for each task, to ensure Mesos making new offers after task completion.
From my perspective, it would be better to fix this problem in Mesos by dropping the constraint on memory for offers, but as temporary solution this patch helps to avoid the dead lock on current Mesos versions.
See [[MESOS-1688] No offers if no memory is allocatable](https://issues.apache.org/jira/browse/MESOS-1688) for details for this problem.

Author: Martin Weindel <martin.weindel@gmail.com>

Closes #1860 from MartinWeindel/master and squashes the following commits:

5762030 [Martin Weindel] reverting work-around
a6bf837 [Martin Weindel] added known issue for issue MESOS-1688
d9d2ca6 [Martin Weindel] work around for problem with Mesos offering semantic (see [https://issues.apache.org/jira/browse/MESOS-1688])
Fix unclosed HTML tag in Yarn docs.