DOC: Clarify index_col behavior for read_csv (pandas-dev#39006)

Author: phofl

doc/source/user_guide/io.rst

@@ -112,8 +112,9 @@ index_col : int, str, sequence of int / str, or False, default ``None``
 
   The default value of ``None`` instructs pandas to guess. If the number of
   fields in the column header row is equal to the number of fields in the body
-  of the data file, then a default index is used.  If it is one larger, then
-  the first field is used as an index.
+  of the data file, then a default index is used.  If it is larger, then
+  the first columns are used as index so that the remaining number of fields in
+  the body are equal to the number of fields in the header.
 usecols : list-like or callable, default ``None``
   Return a subset of the columns. If list-like, all elements must either
   be positional (i.e. integer indices into the document columns) or strings