docs: document --caplin.columns-keep-slots flag (PeerDAS column retention) (#20729)

## Summary

Documents the `--caplin.columns-keep-slots` flag introduced in #20380.

- Adds a new **PeerDAS Data Column Retention** subsection to `caplin.md`
- Flag: `--caplin.columns-keep-slots` (default: 131072, ~18 days)
- Explains use case for DA oracle / rollup nodes needing longer column
history

## Test plan
- [ ] Verify flag appears in `erigon --help` on `release/3.4`
- [ ] Verify default value matches source:
`MIN_EPOCHS_FOR_DATA_COLUMN_SIDECARS_REQUESTS * SLOTS_PER_EPOCH =
131072`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: bloxster <gianni.morselli@erigon.tech>

Author: bloxster

docs/gitbook/src/fundamentals/caplin.md

@@ -27,6 +27,12 @@ In addition, Caplin can backfill recent blobs for an op-node or other uses with
 
 * `--caplin.blobs-immediate-backfill`: Backfills the last 18 days' worth of blobs to quickly populate historical blob data for operational needs or analytics.
 
+### PeerDAS Data Column Retention
+
+For nodes participating in PeerDAS (EIP-7594), Caplin retains data column sidecars for a configurable window:
+
+* `--caplin.columns-keep-slots` (default: `131072`, ~18 days): Number of slots to retain PeerDAS data column sidecars. The default matches `MIN_EPOCHS_FOR_DATA_COLUMN_SIDECARS_REQUESTS × SLOTS_PER_EPOCH`. Increase this value for DA oracle or rollup nodes that require a longer column history.
+
 Caplin can also be used for [block production](../staking/caplin.md), aka **staking**.
 
 ## Beacon API Configuration