pythongh-119003: Clarify slice assignments (python#119935)

Author: nineteendo

Doc/library/stdtypes.rst

@@ -1220,7 +1220,7 @@ accepts integers that meet the value restriction ``0 <= x <= 255``).
 Notes:
 
 (1)
-   *t* must have the same length as the slice it is replacing.
+   If *k* is not equal to ``1``, *t* must have the same length as the slice it is replacing.
 
 (2)
    The optional argument *i* defaults to ``-1``, so that by default the last