Documented services as global vars
[#2865] No change other than moving the parameters section above the services section per a suggestion by @Stof, which I agree with (that parameters are more common to use as global Twig variables)
[#2865] Minor proofreading and tweaks thanks to @xabbuh and @stof about the services as global Twig variables
Merge branch '2.2' into 2.3