docs(rust): document separate decompression scheduler

Record the separate-destination decompression scheduler in the Rust migration
boundary and clarify that C still owns destination naming and private I/O,
resource, and format callbacks.

Test Plan:
- git diff --cached --check

rust/README.md

@@ -120,9 +120,9 @@ zstd ABI:
     adaptive-policy integration, while Rust owns the mixed-format probe/dispatch
     loop, zstd stream-compression I/O loop, scalar adaptive decisions,
     optional-format decompression loops, decompression result policy/final
-    accounting, and both shared- and separate-destination multi-file
-    compression schedulers, plus the shared-destination decompression
-    scheduler. C retains destination-name construction and the private
+    accounting, both shared- and separate-destination multi-file compression
+    schedulers, and both shared- and separate-destination decompression
+    schedulers. C retains destination-name construction and the private
     file/resource/format callbacks for these scheduler boundaries.
     Rust already owns the file preference policy, filename decisions,
     source/destination opening, dictionary buffers, asynchronous I/O pools,
@@ -139,7 +139,7 @@ zstd ABI:
 Dictionary-ingestion dispatch, CCtx dictionary/prefix attachment dispatch, the
 CLI zstd compression stream loop, scalar adaptive decisions, and the
 shared- and separate-destination multi-file compression schedulers,
-shared-destination decompression scheduling,
+shared- and separate-destination decompression scheduling,
 single-threaded stream initialization and the buffered/stable stream state
 machine, MT stream initialization, MT outer scheduling and flush policy, MT
 compression-job stage sequencing and error flow, public sequence-API