Single-point mode incompatible with parallel I/O Netcdf? Suggestion to update documentation.

I recently ported CLM5 to my Linux department's cluster and my MacPro computer with the intention of running a single-point case for the US-UMB Fluxnet site as described in section 1.7.2.1 of the CLM5 User's guide (https://escomp.github.io/ctsm-docs/doc/build/html/users_guide/running-PTCLM/introduction-to-ptclm.html#details-of-ptclmmkdata).

In both machines the case would compile successfully but would not run, with vague log messages (as further detailed here https://github.com/ESMCI/cime/issues/2929#issuecomment-452160826).

I found out that the reason for the crash was the Hdf5 and Netcdf libraries compiled with parallel I/O support. Once I built them with **NO** parallel I/O support or whatsoever, the case ran just fine.

Since I guess many users will try to use the same libraries for single-point cases as they use for regional/global runs (like I did), I think it would be useful to have a note or warning on that issue in the User's Guide, perhaps at the page: https://escomp.github.io/ctsm-docs/doc/build/html/users_guide/running-single-points/running-pts_mode-configurations.html#running-in-a-single-processor.