Auto merge of #39955 - mp4096:master, r=GuillaumeGomez

Docs: Better explanation of return values for min, max functions for the Iterator trait

Added an explanation that `None` is returned if an iterator is empty.

Also added examples for `max` and `min`. I chose not to add examples for other functions like `max_by_key` etc. so that the examples stay concised and focused on the main functionality.