The docs have a different wording, which I suggest copying to the docstring:

   *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.

But now the question remains what the default is -- "full buffering" is
only meaningful with a specified buffer size.  The implementation seems
to default to line buffering.