Document that corpus.json is always written

The save format section predated corpus.json becoming unconditional, so it
described the file as optional and written only when the index has a corpus.
Both are now wrong: the file is always written and it is the contents that are
optional. Also call out that saving without a corpus clears any corpus already
at that path, since that is the part a caller can be surprised by.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: wxiao0421

README.md

@@ -375,18 +375,23 @@ Sentinel supports both local file storage and S3 storage:
 
 The storage is abstracted using `smart_open`, making it seamless to switch between storage backends.
 
-A saved index is a directory of up to three files:
+A saved index is a directory of three files:
 
 | File | Contents |
 |------|----------|
 | `sentinel_local_index_config.json` | Encoder model name, encoding kwargs, model card |
 | `embeddings.safetensors` | The positive and negative embedding tensors |
-| `corpus.json` | The original texts behind those embeddings — **optional** |
+| `corpus.json` | The original texts behind those embeddings — contents **optional** |
 
-`corpus.json` is written whenever the index has a corpus, and it is what lets
-explanations name the matched sentence after a reload. Without it, `explanations`
-falls back to reporting the row number of the match instead of its text. Indices
-saved before this file existed simply lack it and continue to load normally.
+`corpus.json` is what lets explanations name the matched sentence after a reload.
+Without those texts, `explanations` falls back to reporting the row number of the
+match instead of its text.
+
+The file itself is always written, holding nulls when the index has no corpus, so
+that it always describes the embeddings saved beside it. Saving an index without a
+corpus therefore clears any corpus already at that path, rather than leaving
+behind texts that describe rows which no longer exist. Indices saved before this
+file existed simply lack it and continue to load normally.
 
 ## Examples
 To run the notebook examples