Make syntax of specifying namelist fates_history_dimlevel consistent for E3SM and CESM

[rgknox]

The two different host models require this namelist variable to be set in different ways.

For CESM:

fates_history_dimlevel = 2,2

https://github.com/ESCOMP/CTSM/blob/master/cime_config/testdefs/testmods_dirs/clm/FatesColdAllVars/user_nl_clm#L6

For E3SM:

fates_history_dimlevel(1) = 2
fates_history_dimlevel(2) = 2

https://github.com/E3SM-Project/E3SM/blob/e8924b78f9711731c3249c17980550697422b215/components/elm/cime_config/testdefs/testmods_dirs/elm/fates_cold_allvars/user_nl_elm#L5-L6

This confused me, let's try to unify the methods so that others don't get tripped up (and update the documentation as well).

[glemieux]

For reference, the relevant commit with message that states trying to set this in a single line is here: E3SM-Project/E3SM@9be6081

There is also a comment in the PR mentioning this: E3SM-Project/E3SM#6419 (comment).