Trac #16463: random_element for parking functions

implement a fast random parking function. The existing method is *rather
slow*

takes the opportunity to this also refactor the code doc for better
conformance to the coding standards;

URL: http://trac.sagemath.org/16463
Reported by: chapoton
Ticket author(s): Frédéric Chapoton
Reviewer(s): Aladin Virmaux