Improve functors documentation

Quite trivial MR to improve docstrings of functors for their documentation.

• Add documentation for OWNPV-related functors
• Fix misleading track like object definition for some functors (closes #455 (closed), cc @sstahl)
• Explicit difference between IP and IPCHI2 (same for FD), could be more useful in functor composition, see this thread
• A few minor fixes in the returning values of some functors