Issue #7545: improve documentation of the `buffering` argument in io.open().
diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 3308639..adde553 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -106,10 +106,20 @@
:class:`unicode` strings, the bytes having been first decoded using a
platform-dependent encoding or using the specified *encoding* if given.
- *buffering* is an optional integer used to set the buffering policy. By
- default full buffering is on. Pass 0 to switch buffering off (only allowed
- in binary mode), 1 to set line buffering, and an integer > 1 to indicate the
- size of the buffer.
+ *buffering* is an optional integer used to set the buffering policy.
+ Pass 0 to switch buffering off (only allowed in binary mode), 1 to select
+ line buffering (only usable in text mode), and an integer > 1 to indicate
+ the size of a fixed-size chunk buffer. When no *buffering* argument is
+ given, the default buffering policy works as follows:
+
+ * Binary files are buffered in fixed-size chunks; the size of the buffer
+ is chosen using a heuristic trying to determine the underlying device's
+ "block size" and falling back on :attr:`DEFAULT_BUFFER_SIZE`.
+ On many systems, the buffer will typically be 4096 or 8192 bytes long.
+
+ * "Interactive" text files (files for which :meth:`isatty` returns True)
+ use line buffering. Other text files use the policy described above
+ for binary files.
*encoding* is the name of the encoding used to decode or encode the file.
This should only be used in text mode. The default encoding is platform