Make usage of --rotate-pages-threshold clearer

[stegl83]

I waste a lot of time to get the page rotation working.
The problem was, that I used "--rotate-pages-threshold 1" to make page rotation more aggressive. The result was that the page rotation never was performed.

Later than, I found out that I have to use "--rotate-pages --rotate-pages-threshold 1 " to get the desired result.
For me it was not clear, that setting the threshold does NOT implicitly activate page rotation.
Why not?

Even worse, that there is no example in the cookbook nor the usage text of ocrmypdf highlight this behavior.

Please make this at least clearer in the cookbook.