docs/library: Document that sys.version_info returns a 3-tuple only.

See issue #4970.

docs/library/sys.rst

@@ -134,3 +134,9 @@ 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.