Revert flawed doc for slice_after, slice_when, and chunk_while
	 (#5952)

	Restores doc for the methods that were cited in https://bugs.ruby-lang.org/issues/18765.
	---
	 enum.c | 162 +++++++++++++++++++++++++++++++++++++++++++----------------------
	 1 file changed, 107 insertions(+), 55 deletions(-)