Document default parameter of .seek() in the signature. (GH-14691)

(cherry picked from commit 2a3d4d9c)
Co-authored-by: Benjamin Peterson <benjamin@python.org>

Doc/library/io.rst

@@ -304,7 +304,7 @@ I/O Base Classes
       Note that it's already possible to iterate on file objects using ``for
       line in file: ...`` without calling ``file.readlines()``.
 
-   .. method:: seek(offset[, whence])
+   .. method:: seek(offset, whence=SEEK_SET)
 
       Change the stream position to the given byte *offset*.  *offset* is
       interpreted relative to the position indicated by *whence*.  The default
@@ -736,7 +736,7 @@ Text I/O
 
       If *limit* is specified, at most *limit* characters will be read.
 
-   .. method:: seek(offset[, whence])
+   .. method:: seek(offset, whence=SEEK_SET)
 
       Change the stream position to the given *offset*.  Behaviour depends on
       the *whence* parameter.  The default value for *whence* is