The docs for statistics.mean say, "Return the sample arithmetic mean of data, a sequence or iterator of real-valued numbers."  Most of the functions in statistics have a `data` argument, but this function is the only one that says, "a sequence or iterator."  The examples all show lists.

The page would be better if the introduction mentioned what `data` could be, instead of having to guess that median can take an iterator because mean says that it can.

Updated the docs and updated the example codes as well.

Please review :)

Updated patch based on feedback.

Steven, I removed the example of median_grouped using Fractions.
Would you be able to suggest better examples to be added in the docs?

I also noticed that there are more examples of median_grouped in the paragraph below. 

Thanks.

New changeset 71dd21a3b9cc by Raymond Hettinger in branch '3.6':
Issue #27825: Improve for statistics data arguments. (Contributed by Mariatta Wijaya.)
https://hg.python.org/cpython/rev/71dd21a3b9cc

Thanks Mariatta.

As discussed in chat, I dropped the extra fractions examples because they didn't seem to add value.

Sounds good. Thanks, Raymond :)