Expand docs on code and variables in the flow.cylc.

[hjoliver]

I frequently encounter users who still don't have a good grasp of where Jinja2 and shell variables and code can be used in a workflow, and when and where they are evaluated.

This expands on the relevant documentation a bit, and gives some explicit examples.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.

[hjoliver]

@cylc/core - would be good to get this in, if anyone can spare time for a quick review

[jfrost-mo]

Looks like a sensible addition to the documentation. There are a couple minor fixes needed, and I'd also suggesting splitting the examples so they either show valid or invalid use, rather than a mix of both in the same example.

[hjoliver]

All review comments addressed, I think @jfrost-mo

(Note, examples of use of Jinja2 strings that embed shell variables might seem a bit obtuse, but I've seen users trying to do this sort of thing without really understanding what gets evaluated when and where).

[jfrost-mo]

Yep, that reads much more clearly now, thanks.

Only a small additional change requested to link a couple glossary terms, including a new definition of "run host".

[hjoliver]

Comments addressed.

[jfrost-mo]

Looking good.