#15840: make docs consistent by saying operations on closed files raise ValueError.

Patch by Caelyn McAulay.

Neither Caelyn nor I could find any cases in 2.7 or 3.4/5 where an
operation on a closed stream raised IOError; generally the C
implementations have a macro to check for the stream being closed, and
these macros all raised ValueError.  If we find any, a new bug should
be opened.