[3.12] gh-107801: Improve the accuracy of io.TextIOWrapper.seek docs (#107933) (#108262)

(cherry picked from commit 7f87ebbc3f52680c939791f397b9a478edf0c8d4)

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