Update README files to clarify feature flags and build strategies for fdb-sys and fdb crates

Author: Choochmeque

rust/crates/fdb-sys/README.md

@@ -6,11 +6,36 @@ This crate provides raw FFI bindings using [cxx](https://cxx.rs/). For a safe, e
 
 ## Features
 
-- `vendored` (default) - Build FDB5 and dependencies from source
-- `system` - Link against system-installed FDB5
-- `grib` - GRIB format support via ecCodes
-- `tocfdb` - Filesystem TOC support
-- `fdb-remote` - Remote FDB access
+### Build strategy (mutually exclusive)
+
+- `vendored` - Build the FDB and its dependencies (eckit, metkit, ecCodes) from source.
+- `system` - Link against system-installed FDB.
+
+Note: neither is enabled by default on `fdb-sys` itself. End users should
+depend on the higher-level [`fdb`](https://crates.io/crates/fdb) crate, which
+defaults to `vendored`. If you depend on `fdb-sys` directly you must select
+one explicitly.
+
+### Core (enabled by default)
+
+- `grib` - GRIB format support. Pulls in `eccodes-sys/product-grib` and
+  `metkit-sys/grib` so the GRIB message splitter is registered with
+  `eckit::message::Splitter`.
+- `tocfdb` - Filesystem TOC backend (the standard local FDB store).
+- `fdb-remote` - Client support for remote FDB servers.
+
+### Storage backends (off by default; require external libraries)
+
+- `radosfdb` - Ceph/RADOS object store backend (requires RADOS).
+- `lustre` - Lustre file striping control (requires LUSTREAPI).
+- `daosfdb` - DAOS object store backend (requires DAOS).
+- `daos-admin` - DAOS pool management (requires DAOS).
+- `dummy-daos` - Filesystem-emulated DAOS (no DAOS install needed).
+
+### Other (off by default)
+
+- `experimental` - Experimental upstream features.
+- `sandbox` - Sandbox builds.
 
 ## License
 

rust/crates/fdb/README.md

@@ -33,8 +33,14 @@ reader.read_to_end(&mut results)?;
 
 ## Features
 
-- `vendored` (default) - Build FDB5 and dependencies from source
-- `system` - Link against system-installed FDB5
+- `vendored` (default) - Build the FDB and its dependencies (eckit, metkit,
+  ecCodes) from source.
+- `system` - Link against a system-installed FDB.
+
+Lower-level feature flags (GRIB support, storage backends, experimental
+features) live on the [`fdb-sys`](https://crates.io/crates/fdb-sys) crate;
+see its README for the full list. The defaults inherited here enable GRIB,
+the filesystem TOC backend, and remote FDB client support.
 
 ## Running