I noticed a few typos while reading the documentation while preparing for #570, so I fixed them.
Seeing as it is probably best to wait with working on #570 until develop branch passes unit tests, and these changes are separate from that issue anyway, I thought it might be better to open a separate PR.

There were some other PRs for minor documentation improvements (e.g. #561), so I thought this was appropriate.

There is nothing to test as all changes are in documentation (either code comment or rst).