Describe omitted slice parameters.

Author: Alex-Wasowicz

Doc/reference/datamodel.rst

@@ -325,6 +325,12 @@ Some sequences also support "extended slicing" with a third *step* parameter:
 ``i = start + n*step`` and ``0 <= n``, such that ``start <= i < stop`` for 
 ``0 <= step``, and ``stop < i <= start`` for ``step < 0``.
 
+Slicing parameters can be omitted:
+* When *step* is omitted, it defaults to 1.
+* When *start* is omitted, the selection starts from the 1st index depending on
+the *step* direction (index 0 for ``0 < step``, subscript -1 for ``step < 0``).
+* When *end* is omitted, the selection ends when there isn't any element left.
+
 Sequences are distinguished according to their mutability: