User's Guide: Update instructions for generating crop maturity reqts

[samsrabin]

What sort(s) of documentation issue is this?

• Something is missing.
• Something is (or might be) incorrect or outdated.
• Something is confusing.
• Something is broken.

What part(s) of the documentation does this concern?

• Technical Note (science and design of the model)
• User's Guide (using the model and related tools)
• Somewhere else (e.g., README file, tool help text, or code comment): Please specify
• I don't know

Describe the issue

The instructions for generating crop maturity requirements need some additional lines in user_nl_clm:

model_year_align_cropcal_swindows = YEAR
stream_fldFileName_cultivar_gdds = ''
stream_fldFileName_gdd20_baseline = ''
cropcals_rx_adapt = .false.
cropcals_rx = .true.

Also, consider adding instructions for custom surface dataset with all crops everywhere:

fsurdat = '/glade/work/samrabin/fsurdat_all_crops_everywhere/surfdata_360x720cru_hist_78pfts_CMIP6_simyr1850_c230201.all_crops_everywhere.netCDF-3.nc'
do_transient_crops = .false.
flanduse_timeseries = ''
use_init_interp = .true.

[samsrabin]

Requires some Python code changes; see https://github.com/samsrabin/CTSM/tree/new-maturity-reqs-202507.

[samsrabin]

Probably do #3664 first.