DOC: update docstring of DataFrame.append
I always thought that `append` was for appending rows, not columns. So, unless I am completely missing something, the docstring was saying the wrong thing.

So I reworded it a bit, see below. 
Apart from that, there were some other things that are not fully clear to me in the docstring:
- The docstring says that `other` should be a "list of Series/dict-like objects" (or a Dataframe):
  - Does it need to be a list? It also works with just a Series. Or what are the differences between both?
  - With a simple example, I don't see a difference, although in implementation, there is.
- The docstring says for the `ignore_index` kwarg: "Useful for gluing together record arrays"
  - How does this look in practice? If I try to append a record array, I get "TypeError: cannot concatenate a non-NDFrame object" -> so I will remove this
