#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.

Modules/_io/iobase.c

@@ -42,8 +42,8 @@ PyDoc_STRVAR(iobase_doc,
     "bytes. bytearrays are accepted too, and in some cases (such as\n"
     "readinto) needed. Text I/O classes work with str data.\n"
     "\n"
-    "Note that calling any method (even inquiries) on a closed stream is\n"
-    "undefined. Implementations may raise IOError in this case.\n"
+    "Note that calling any method (except additional calls to close(),\n"
+    "which are ignored) on a closed stream should raise a ValueError.\n"
     "\n"
     "IOBase (and its subclasses) support the iterator protocol, meaning\n"
     "that an IOBase object can be iterated over yielding the lines in a\n"