Add description of `scale` to declarative tutorial
Add description of `scale` to declarative tutorial

Fixes #2136

This is essentially a copy/paste of the docstring into the tutorial
document.

Unable to test due to errors below, but this is trivial so hopefully
it works!

```
preparing documents... WARNING: unsupported theme option 'navbar_start' given
WARNING: unsupported theme option 'navbar_center' given
WARNING: unsupported theme option 'navbar_end' given
done
writing output... [  0%] api/generated/metpy.calc
Theme error:
An error happened in rendering the page api/generated/metpy.calc.
Reason: TemplateNotFound('versions')
```

Unidata/MetPy,Unidata/MetPy