Add notes about s[i] in Common Sequence Operations

Documentation

Operation	Result	Notes
`s[i]`	ith item of s, origin 0	(3)

If i or j is negative, the index is relative to the end of sequence s: len(s) + i or len(s) + j is substituted. But note that -0 is still 0.

We can add a note: If the sequence is empty or i is outside the sequence range, IndexError will be raised.