Update difflib.py

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Author: donbarbos

Lib/difflib.py

@@ -1040,7 +1040,7 @@ def _qformat(self, aline, bline, atags, btags):
 
 def IS_LINE_JUNK(line, pat=None):
     r"""
-    Return True for ignorable line: if `line` is blank or contains a single '#'.
+    Return True for ignorable line: if and only if `line` is blank or contains a single '#'.
 
     Examples: