Element vs program vs...: confusion in the docs

We have some confusion in the documentation (and in the code) in that we have a number of terms that are somewhat the same, but not quite, and we are inconsistent in our use of them.

They are:

• Element

• Program

• Component

• Runnable

At certain times, they have all been used to refer to the same thing: a Flow!
But we need to clarify:

• exactly what they mean;

• which ones we want to use;

• when to use them;

• clean up their use in both the docs and in the code (as these get used in CLI doc strings); and

• add them all to the Glossary.