add description to flandusepftdat definition

bld/namelist_files/namelist_definition_ctsm.xml

@@ -826,7 +826,13 @@ types to vary over time.
 
 <entry id="flandusepftdat" type="char*256" category="datasets"
        input_pathname="abs" group="clm_inparm" valid_values="" >
-Full pathname of fates landuse x pft static data map.
+Full pathname of fates landuse x pft association static data map.  
+The file associates land use types with pfts across a static global map.
+This file is necessary for running FATES with use_fates_luh, 
+use_fates_nocomp, and use_fates_fixedbiogeo engaged.  The file is output
+by the FATES land use data tool (https://github.com/NGEET/tools-fates-landusedata)
+which processes the raw land use data from the THEMIS tool data sets
+(https://doi.org/10.5065/29s7-7b41)
 </entry>
 
 <entry id="use_luna" type="logical" category="physics"