Missing yo-doc for some zefops

[ageron]

The following zefops have no yo documentation:

• yo(L)
• yo(allow_tombstone)
• yo(expect)
• yo(collect)
• yo(func)

[zeyadkhaled]

• L is a relic from old traversal styles and should be removed altogether if there are no dependencies on it anymore.

• allow_tombstone and expect were both throwing an exception when yo was called on them, that has been fixed now, but they will remain for the time being without a docstring as they don't have any implementations but are used as flags, I think these should be revisited and changed to ValueTypes as they don't have any implementations. Would you agree @pengwyn?

• Both collect and func had an ugly repr and that has been fixed now. As for showing a docstring, it would be tricky as they aren't exactly a ZefOp. But it is something I can play around with at some point.

[pengwyn]

I agree with making those allow_tombstone and expect turned into ValueTypes. Although expect I don't think has been used yet, so we might have to rejig that for a different purpose later on.