updated ellipsis chars

As discussed in #8593, here are changes to documentation to use midline ellipsis ⋯ to refer to etc., to distinguish from the splice notation ... (three periods in a row). It was confusing because the same string of characters had two different meanings, which could only be distinguished by guessing the author's intent. I believe that ⋯ was intended in the majority of cases, and I have attempted to preserve explicit correct use of splice (...) where appropriate. 

I am new to Julia. I would appreciate if someone could look through all of the many changes to verify their correctness.