JuliaLang#23788

`See also`-Lists now contain up to two related functions and antonym at the end

Docs of `append!` were not in line with `prepend!`.

I think the sentence is more clear, with out the "which are not already..." part.

I removed all descriptions of other functions and inserted an entry in `See also:` instead.
This is related to my first commits' proposal of using a link-description list as `See also:`. 

I also removed the redundant description in `splice!` for removing items. I opted to remove it from the single-item method, as a (negative) range is used to remove items.