Issue #20699: Document that “io” methods accept bytes-like objects

This matches the usage of ZipFile and BufferedWriter. This still requires
return values to be bytes() objects.

Also document and test that the write() methods should only access their
argument before they return.

Modules/_io/iobase.c

@@ -53,8 +53,9 @@ PyDoc_STRVAR(iobase_doc,
     "called.\n"
     "\n"
     "The basic type used for binary data read from or written to a file is\n"
-    "bytes. bytearrays are accepted too, and in some cases (such as\n"
-    "readinto) needed. Text I/O classes work with str data.\n"
+    "bytes. Other bytes-like objects are accepted as method arguments too.\n"
+    "In some cases (such as readinto), a writable object is required. Text\n"
+    "I/O classes work with str data.\n"
     "\n"
     "Note that calling any method (except additional calls to close(),\n"
     "which are ignored) on a closed stream should raise a ValueError.\n"