[doc] Update pickle protocol doc in config2.py

Change-Id: If1dcd91cbf704895b9e04a6bbc821d8affab1a9b
---
M pywikibot/config2.py
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/pywikibot/config2.py b/pywikibot/config2.py
index b5e5a1c..79860ac 100644
--- a/pywikibot/config2.py
+++ b/pywikibot/config2.py
@@ -879,9 +879,12 @@
 
 # Pickle protocol version to use for storing dumps.
 # This config variable is not used for loading dumps.
+# Version 0 is a more or less human-readable protocol
 # Version 2 is common to both Python 2 and 3, and should
 # be used when dumps are accessed by both versions.
-# Version 4 is only available for Python 3.4
+# Version 3 is only available for Python 3
+# Version 4 is only available for Python 3.4+
+# Version 5 was added with Python 3.8
 pickle_protocol = 2
 
 # ============================

