Update documentation

docs/configuration/experimental/cache-file.md

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+---
+icon: material/new-box
+---
+
+!!! question "Since sing-box 1.8.0"
+
+### Structure
+
+```json
+{
+  "enabled": true,
+  "path": "",
+  "cache_id": "",
+  "store_fakeip": false
+}
+```
+
+### Fields
+
+#### enabled
+
+Enable cache file.
+
+#### path
+
+Path to the cache file.
+
+`cache.db` will be used if empty.
+
+#### cache_id
+
+Identifier in cache file.
+
+If not empty, configuration specified data will use a separate store keyed by it.