prune: more human-friendly documentation The previous help string generally had a highly technical language. I've tried my best to address that.

prune: improve help first line The previous help string for prune didn't mention successors in the first line. Note: Pierre-Yves David changed the original "hidden" to "obsolete" as it is better concept to put forward. (a bit more specific and with more consequence than just "hidden"). We need a good `hg help obsolete/obsolescence` result. We'll try to get this in the next version.