Add limited options for some dataset options and improve explanations

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

Start by tracing where dataset run metadata options such as model_id, calendar, experiment_id, suite_id, and variant_label are defined and consumed. Check whether Rose metadata or a central run-metadata list already provides the accepted values. Done means applying limited options where appropriate, improving the help explanations, and removing “For example” when the controlled list makes it unnecessary.

Written by the indexing model from the issue text.