bpo-39693: mention KeyError in tarfile extractfile documentation (GH-18639) 

Co-authored-by: Andrey Darascheka <andrei.daraschenka@leverx.com>
(cherry picked from commit ec42789e6e14f6b6ac13569aeadc13798d7173a8)

Closes bpo-39693.
diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 6769066..62a6d9d 100755
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -2091,9 +2091,10 @@
 
     def extractfile(self, member):
         """Extract a member from the archive as a file object. `member' may be
-           a filename or a TarInfo object. If `member' is a regular file or a
-           link, an io.BufferedReader object is returned. Otherwise, None is
-           returned.
+           a filename or a TarInfo object. If `member' is a regular file or
+           a link, an io.BufferedReader object is returned. For all other
+           existing members, None is returned. If `member' does not appear
+           in the archive, KeyError is raised.
         """
         self._check("r")