EAMxx: document the need for UNLIMITED time dim

Author: mahf708

components/eamxx/docs/user/nudging.md

@@ -9,6 +9,7 @@ The user is expected to prepapre (and then use `atmchange` to point to) nudging
 In practice, this means that the data files must contain variable names known to EAMxx (only U, V, T_mid, and qv are supported now).
 The files can be specified with an explicit list or via pattern matching.
 The files must contain an arbitary global attribute `case_t0`, and it is recommended to be the same as the time dimension unit (the files must be time-dimensioned).
+The time dimension should be UNLIMITED.
 Finally, the dimension order must be such that `lev` is the last dimension, so most likely, the user must transpose the dimensions.
 
 ## Pressure in the nudging data