bpo-39369 Doc: Update mmap readline method documentation (GH-17957)

* Update mmap readline method documentation

Update mmap `readline` method description. The fact that the `readline` method does update the file position should not be ignored since this might give the impression for the programmer that it doesn't update it.

* 📜🤖 Added by blurb_it.

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>