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>
diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
index 12b14d6..1f3fbc3 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -244,7 +244,8 @@
    .. method:: readline()
 
       Returns a single line, starting at the current file position and up to the
-      next newline.
+      next newline. The file position is updated to point after the bytes that were
+      returned.
 
 
    .. method:: resize(newsize)