Document absoluteness of sys.executable

Closes #13402.
diff --git a/Misc/NEWS b/Misc/NEWS
index aa05096..0f0479c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -454,6 +454,8 @@
 Documentation
 -------------
 
+- Issue #13402: Document absoluteness of sys.executable.
+
 - Issue #13883: PYTHONCASEOK also used on OS X and OS/2.
 
 - Issue #12949: Document the kwonlyargcount argument for the PyCode_New