Document template variables rose edit section

### What problem does your feature request solve?

The template variables section shows up in rose edit if a variable is added to the configuration without being put into another section, usually because it is no longer used. These can be confusing, as they often have no help text or other context.

### Describe the solution you'd like

Configuration in the template variables section is harmless, as it will have no effect. We should document it in the sphinx docs so users are not confused.

### Describe alternatives you've considered

I'm a little concerned about the discoverability of this documentation, but hopefully the search will be sufficient.

Start by locating the Sphinx documentation covering rose edit and template variables. Read the surrounding explanation of configuration sections, then document that variables left in the template variables section are harmless and have no effect when unused; done means users can find and understand this behavior.

Written by the indexing model from the issue text.