docs: correct three factual errors in the provenance documentation

Second-pass review against the API source found three claims that were
wrong rather than merely loose.

NDX was listed as an example of index_feed. It is not in IndexSymbols, so
SymbolClassifier falls it through to equity - an NDX request ticks
equity_feed, not index_feed. The other services in the API do treat NDX as
an index, which is what made the claim look safe. Now names only roots
that are actually classified as indexes.

data_as_of on the replay service was described as "all null". Every feed
is null, but node is always populated - it identifies which node answered.
"All null" would have a caller testing the wrong thing.

The historical SDKs repeated the live line about bare JSON arrays carrying
the envelope in X-Data-As-Of and X-Endpoint-Version headers. That does not
hold there: the header path never calls Additional(), so archive_as_of -
the only provenance that means anything on a replay host - is not emitted
at all, and the historical service has no bare-array endpoints in the
first place. The claim is dropped rather than reworded, since the case
does not arise.

The nine feed names and their order were verified against FeedClock and
ReplayClock in all ten SDKs: every one matches, order included.

Author: tdobrowolski1

README.md

@@ -55,7 +55,7 @@ stamps: DataAsOf = gex["data_as_of"]
 | `node` | Which node answered | Nodes hydrate independently |
 | `equity_feed` | Equity and ETF spot quotes | seconds, during market hours |
 | `equity_options_feed` | Equity and ETF option quotes | seconds, during market hours |
-| `index_feed` | Index spot (SPX, NDX, RUT, VIX) | seconds, during market hours |
+| `index_feed` | Index spot (SPX, RUT, VIX and the other index roots) | seconds, during market hours |
 | `index_options_feed` | Index option quotes | seconds, during market hours |
 | `futures_feed` | Futures prices | seconds, during the futures session |
 | `futures_options_feed` | Futures option quotes | seconds, during the futures session |