DOC: clarify purpose of DataFrame.from_csv (GH4191)
Closes #9556

xref #9568, #4916, #4191 

However, while writing this up, I started to doubt a bit if this is necessary. 
`DataFrame.from_csv` is implemented as a round-trip method together with `to_csv`. If you use a plain `df.to_csv(path)`, you cannnot read it in as `pd.read_csv(path)` to get exactly the same. You at least need `pd.read_csv(path, index_col=True)` and a `parse_dates` keyword if you have datetimes. 

Secondly, there is also the question of `Series.from_csv`. It would be logical to deprecate this as well, but for this you don't directly have an alternative (but `pd.read_csv(path, index_col=0)[0]` will work). There is also for example this SO answer of Wes: http://stackoverflow.com/questions/13557559/how-to-write-read-pandas-series-to-from-csv (and it is used in the Python for Data Analysis book).

`pd.read_csv(path, index_col=0, squeeze=True)` should work for `Series.from_csv`.

I am also not strong on actually deprecating this particular method (as you say its the inverse of `to_csv`).
 let's push this off / close. Not sure this is necessary.
 let's just update the docs a tiny bit to note that `DataFrame.to_csv <-> DataFrame.from_csv` (and put it on `pd.read_csv` somewhere? I don't think we can actually deprecate this
 Yes, it was on my to do list to update this PR with only the docs part (and not the actual deprecation). 
I think we can still encourage users in the docstring to use `read_csv`, but note that `from_csv` is useful if you want an exact opposite of `to_csv`
 which is actually the part from the docstring that I now removed ... :-)
 @jreback updated it. Thoughts?

I still have to do the same for `Series.from_csv`
 lgtm. DISCOURAGED should be an official term!
 lgtm
 updated with similar text for Series.from_csv
