[CMake] help texts

[Rolf Eike Beer]

> words, the markers are doubled, e.g. **bold words**.
>
> * lists formatting. E.g:
>
>   - bullet list
>   * another bullet item
>   ** nested bullet list
>   *** deeply nested list

How to specify bold words in the second bullet item? Simple solution would
be to force list and title specifiers to always be at the beginning of a
line (with no spaces before them).

> * section titles. I would go for something like this:
>
>   = Title
>   == Sub-Title
>   === Sub-Sub-Title

Just a thought: one could take the MediaWiki style and require the same
symbol be after the text, e.g. = Title =.

Eike