Small improvements to str_cat
closes #11435 
closes #11334.

Added some doc examples to make the behavior with `nan` more clear, and catch an error condition when a `sep` is passed instead of an `others` (array) as an argument and print a more intuitive error message.

tests!
 pls also add entries in whatsnew (one for each of the bug fixes). we'll call ignoring nans a bug fix.
 can you rebase / update
 I may have just botched this... tried rebasing on master and now there are many changes I didn't include reflected in the diff. 
 http://pandas.pydata.org/pandas-docs/stable/contributing.html#contributing-your-changes-to-pandas

`git rebase -i master`

then

`git push <yourremote> <yourbranch> -f`
 You have to make sure your _your_ master is in sync with upstream master. 

You can revert to your original patch with "git reset --hard HASH" -- look in your git reflog to find the commit hash. 
 This was botched and left to languish-- have restarted and will ref this in the re-do.
