[Flang][Docs] Fix warnings when building docs.

Following warning were seen with recommonmark(0.5.0) and sphinx(1.8.5).
`parser.py:75: UserWarning: Container node skipped: type=document warn("Container node skipped: type={0}".format(mdnode.t))`

The warnings are due to an issue in recommonmark's(a python package) older versions.
A better solution is to use the latest version of recommonmark(>=0.6.0) to avoid
these issue in the first place.
This patch fixes the warnings for older versions.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D91117