docs(skiplist): update insert function document

[Tianion]

"it will be removed before inserting the new one", it looks like there are two steps involved in the process described. #1122

[taiki-e]

Thanks. It seems to me that “using CAS” is a description of implementation details and does not explain what specific behavior is provided. (I guess you want to document the semantics that no intermediate state is observed during the update.)