Document contao.image.imagine_service more clearly

[fritzmg]

Currently there is only an explanation on how to figure out which service is used currently. And there is of course the config reference dump showing that you can override the imagine service. However, I think it should be documented more clearly, that you can do this, and more importantly, which options you have:

• contao.image.imagine.gd
• contao.image.imagine.gmagick (untested)
• contao.image.imagine.imagick (untested)