Update User's Guide: Running CLM5.0 with irrigation

The ["Running with irrigation"](https://github.com/ESCOMP/CTSM/blob/master/doc/source/users_guide/running-special-cases/running-with-irrigation.rst#running-with-irrigation) section of the User's Guide contains incorrect instructions for CLM5.0. As described by @olyson in [this DiscussCESM forum reply](https://bb.cgd.ucar.edu/cesm/threads/issue-using-irrigation-in-model-run.5173/post-35953), the proper way to turn on irrigation is to set `irrigate = .true.` in the `user_nl_clm` namelist.

My suggestion is to have three separate subsections here, one for each of 4.0, 4.5, and 5.0.

Relevant to #509. Once that's implemented, the instructions for 5.0 will need to specify that irrigation is actually on by default for crop runs, but it can be turned off via the namelist.