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.
{"site_id":"cf023865-0eb1-49e9-951b-29555949abf0","path":"html/"}