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>

Author: Pardim93

Doc/library/mmap.rst

@@ -244,7 +244,8 @@ To map anonymous memory, -1 should be passed as the fileno along with the length
    .. 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)

Misc/NEWS.d/next/Documentation/2020-01-17-13-59-21.bpo-39369.Bx5yE3.rst

@@ -0,0 +1 @@
+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.