Improved documentation

Hi, during reading I observed some errors in the documentation and fixed them now. This includes

- typos
- Unwanted indentation of code examples
- Missing option descriptions
- Avoidance of list items started with a blank line in PDF output (caused by `indexterm` before `para`).

I'd be happy if my changes would be merged.

Attention: I built the PDF myself but got the following warnings:

```
Don't know what gentext to create for xref to: "listitem" (linkend=item-default-backend)
Don't know what gentext to create for xref to: "listitem" (linkend=item-default-backend)
```
So you may have a closer look at this.