Update sample_inverted_index.md

docs/src/cli/sample_inverted_index.md

@@ -5,3 +5,38 @@
 ```
 <!-- cmdrun ../../../build/bin/sample_inverted_index --help -->
 ```
+
+## Description
+
+Creates a smaller inverted index from an existing one by sampling postings or
+documents. The purpose of this tool is to reduce time and space requirements
+while preserving the main statistical properties of the original collection,
+making it useful for faster experiments and debugging.
+
+### Sampling strategy (`-t, --type`)
+
+- `random_postings`: keep random occurrences per posting list (not whole
+  posting lists).
+- `random_docids`: keep all postings belonging to a random subset of documents.
+
+## Examples
+
+### Keep ~25% of postings
+
+```bash
+sample_inverted_index \
+    -c path/to/inverted \
+    -o path/to/inverted.sampled \
+    -r 0.25 \
+    -t random_postings
+```
+
+### Keep ~25% of the documents
+
+```bash
+sample_inverted_index \
+    -c path/to/inverted \
+    -o path/to/inverted.sampled \
+    -r 0.25 \
+    -t random_docids
+```