[cleanup] fix some typos in documentation strings

I still had some old commits lying around that fixed some typos in comments. Since then I found the codespell program which turned out to be quite helpful in finding common misspellings.

So I guess, I've gathered enough fixes by now to justify a merge request.

Similar merge requests for the rest of the Dune core modules will follow soon.

I've now submitted similar merge requests for the following repositories:

• dune-geometry!14 (merged)
• dune-grid!44 (merged)
• dune-istl!37 (merged)
• dune-localfunctions!11 (merged)
• staging/dune-functions!17 (merged)
• pdelab/dune-typetree!9

mentioned in commit 63413211

Status changed to merged

Thanks for the polish. The merge requests for functions and TypeTree will hopefully be accepted by people with the according rights.

Thanks for responding quickly as usual.

Mentioned in commit 63413211