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).
