Use `# Extended help` sections where useful/helpful/applicable

I just came across the feature of docstrings that allows one to access an *extended help* from the REPL help mode. This works such that if you add a section labelled `# Extended help` in your docstring, it is hidden by default when you query the help, e.g., by typing `?trixi_include`. In lieu of that section, it will just show a hint to the user
```
Extended help is available with `??`
```
If one instead types `??trixi_include` in the REPL, the original docstring + the extended help section is shown.

I do not have a specific use case, I just wanted to bring this to attention since it might allow us to (re-)structure longish docstrings in the future.

**EDIT**: [Here](https://docs.julialang.org/en/v1/manual/documentation/#Writing-Documentation) is the relevant part of the Julia docs, see point 11.