Add limited options for some dataset options and improve explanations

[NParsonsMO]

e.g.:

model_id should be a controlled list (CDDS uses this). We can then remove "For example". Use the help to tell users why we need this.

Investigate:

• Where are the different options for datasets used?
• Whether they come from a list of known options

Time box:

• 0.5 day

For example:
[namelist:model_runs(reference)]
calendar="360_day" - CDDS
experiment_id="historical" - ESMValTool
label_for_plots="HadGEM3-GC3.1 N96ORCA1" [Possibility to add info on model horizontal and vertical resolution info] - ESMValTool & AutoAssess
model_id="HadGEM3-GC31-LL" - CDDS & ESMValTool
suite_id="u-bv526" - CDDS & ESMValTool & AutoAssess (& CMEW behaviour)
variant_label="r5i1p1f3" - ESMValTool

Consider ordering this list based on usage.

Possible solutions:

• Add the acceptable entries in to the Rose metadata.
• If there is a central list of run metadata, use the URL