I think, as a whole, your changes are an improvement and make it easier to understand this concept.

If a countth substring

I personally stumbled on this though. It took me a few re-reads to understand this is talking about "substring with countth number of items". I can't think of a better suggestion at the moment without making it too verbose. Going to think about this...

For non-english reader, this is particurlarly hard.

How about this:

If count substrings are returned, the last substring i.e. the countth substring will contain the remainder of the string.

I think that's better. Or maybe even:

If count substrings are returned, the last substring (the countth substring) will contain the remainder of the string.

What do you think @GuillaumeGomez ?

Crazy idea: rename the parameter from count to n (slice::splitn's parameter is called n). Then we can say "the nth substring" which is probably easier to parse.

@frewsxcv: I prefer @durka's proposition. :)

I'm generally against single letter variable names, but considering the method name is splitn, I think n makes sense. What do you think @angelsl? If you're alright with it, can you rename the parameter and update the instances of count in the doc comment?

Sounds good to me. I've amended the commit.