Fix documentation in SparseSetArray.

"n" in SparseSetArray is actually the key, not the index. "index" is used
as a parameter name in this class wherever it actually is the index.
Using index in documentation in the wrong places gives users the wrong
understanding.

Bug: N/A
Test: N/A
Change-Id: I239a653347dab4ea1a00c6d709aac06a98012a46