Improve documentation re. variable names in HISTORY.rc

[gmao-rreichle]

Users often stumble over the difference in variable names when GEOSldas is run in ensemble mode as opposed to single-member/no-perturbations mode. Output of the layer-1 soil temperature, e.g., uses TP1 or TSOIL1TILE in HISTORY.rc, depending on whether or not GEOSldas runs in ensemble mode.