Improve parameter descriptions

[samsrabin]

At the definition of many parameters in pftconMod, many have only generic descriptions that do not help with understanding of the code. E.g.: https://github.com/ESCOMP/CTSM/blob/1e2e2c35d9568c94eac6cb606f37c18294158682/src/main/pftconMod.F90#L178-L183

These comments should be replaced with the long names from the parameter file, which are more descriptive.