gh-107801: Improve the accuracy of io.TextIOWrapper.seek docs (#107933)

Clearly document the supported seek() operations:

- Rewind to the start of the stream
- Restore a previous stream position (given by tell())
- Fast-forward to the end of the stream

Author: erlend-aasland

Doc/library/io.rst

@@ -1044,6 +1044,22 @@ Text I/O
       .. versionchanged:: 3.11
          The method supports ``encoding="locale"`` option.
 
+   .. method:: seek(cookie, whence=os.SEEK_SET, /)
+
+      Set the stream position.
+      Return the new stream position as an :class:`int`.
+
+      Four operations are supported,
+      given by the following argument combinations:
+
+      * ``seek(0, SEEK_SET)``: Rewind to the start of the stream.
+      * ``seek(cookie, SEEK_SET)``: Restore a previous position;
+        *cookie* **must be** a number returned by :meth:`!tell`.
+      * ``seek(0, SEEK_END)``: Fast-forward to the end of the stream.
+      * ``seek(0, SEEK_CUR)``: Leave the current stream position unchanged.
+
+      Any other argument combinations are invalid,
+      and may raise exceptions.
 
 .. class:: StringIO(initial_value='', newline='\n')
 

Modules/_io/clinic/textio.c.h

@@ -2428,13 +2428,29 @@ _textiowrapper_encoder_setstate(textio *self, cookie_type *cookie)
 /*[clinic input]
 _io.TextIOWrapper.seek
     cookie as cookieObj: object
-    whence: int = 0
+      Zero or an opaque number returned by tell().
+    whence: int(c_default='0') = os.SEEK_SET
+      The relative position to seek from.
     /
+
+Set the stream position, and return the new stream position.
+
+Four operations are supported, given by the following argument
+combinations:
+
+- seek(0, SEEK_SET): Rewind to the start of the stream.
+- seek(cookie, SEEK_SET): Restore a previous position;
+  'cookie' must be a number returned by tell().
+- seek(0, SEEK_END): Fast-forward to the end of the stream.
+- seek(0, SEEK_CUR): Leave the current stream position unchanged.
+
+Any other argument combinations are invalid,
+and may raise exceptions.
 [clinic start generated code]*/
 
 static PyObject *
 _io_TextIOWrapper_seek_impl(textio *self, PyObject *cookieObj, int whence)
-/*[clinic end generated code: output=0a15679764e2d04d input=0458abeb3d7842be]*/
+/*[clinic end generated code: output=0a15679764e2d04d input=0f68adcb02cf2823]*/
 {
     PyObject *posobj;
     cookie_type cookie;