docs/library: Document that sys.version_info returns a 3-tuple only.
See issue
@@ -121,3 +121,9 @@ Constants
.. data:: version_info
Python language version that this implementation conforms to, as a tuple of ints.
+
+ .. admonition:: Difference to CPython
+ :class: attention
+
+ Only the first three version numbers (major, minor, micro) are supported and
+ they can be referenced only by index, not by name.
