[GHC] #9358: Improve flag description in the user guide

[Jan Stolarek]

>  Jan, you said you'd be willing to do this updating before 7.10
Yes, that's the plan. The question is do I have to make it before the mid-November feature freeze? 
Austin, what's your opinion?
 
>  So far as I can see, there simply is no list of what is enabled by -O or
>  -O2.
I can make such a list if you want.

>  It's more than a simple on/off  thing because the order matters.
Hmm.... in which cases does it matter? Can you give an example?

Janek