Update extensions tutorials/doc

[rly]

Motivation

Fix #1291.

The extensions documentation could use some more work, but at least this fixes the code examples and adds important information.

Checklist

• [x] Have you checked our Contributing document?
• [x] Have you ensured the PR description clearly describes problem and the solution?
• [x] Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• [x] Have you checked to ensure that there aren't other open Pull Requests for the same change?
• [x] Have you included the relevant issue number using #XXX notation where XXX is the issue number?