Add manifest caching notes to docs

Author: kris-gaudel

mkdocs/docs/configuration.md

@@ -48,6 +48,21 @@ The environment variable picked up by Iceberg starts with `PYICEBERG_` and then
 
 For example, `PYICEBERG_CATALOG__DEFAULT__S3__ACCESS_KEY_ID`, sets `s3.access-key-id` on the `default` catalog.
 
+## Manifest Caching
+
+PyIceberg caches `ManifestFile` objects locally and uses an LRU policy to bound the cache size. By default, up to `128`
+manifest entries are retained.
+
+You can change the cache size with the `PYICEBERG_MANIFEST_CACHE_SIZE` environment variable:
+
+```sh
+export PYICEBERG_MANIFEST_CACHE_SIZE=256
+```
+
+The memory used by this cache depends on the size and number of distinct manifests your workload touches. Lower the value
+if you want a tighter memory bound, or call `clear_manifest_cache()` to proactively release cached manifest metadata in
+long-lived processes. Setting `PYICEBERG_MANIFEST_CACHE_SIZE` to `0` disables manifest caching entirely.
+
 ## Tables
 
 Iceberg tables support table properties to configure table behavior.