docs(rust): clarify decoder context ownership

Clarify that the decoder's private C layout remains configuration-sensitive in
C while Rust owns storage allocation, state-machine behavior, and the public
ABI. This keeps the module-level migration boundary consistent with the
allocator bridge now implemented in Rust.

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

rust/src/zstd_decompress.rs

@@ -6,10 +6,11 @@
 
 //! Frame, context, and streaming decompression orchestration.
 //!
-//! `ZSTD_DCtx` deliberately remains C-owned.  The companion C translation
-//! unit projects its build-configuration-dependent leaves into
-//! [`ZSTD_rustDctxView`]; this module owns the decoder state machine and
-//! public ABI while never assumes offsets for the private C context.
+//! The private `ZSTD_DCtx` layout deliberately remains C-owned.  The companion
+//! C translation unit projects its build-configuration-dependent leaves into
+//! [`ZSTD_rustDctxView`], while this module owns allocation, the decoder state
+//! machine, and the public ABI without assuming offsets for the private
+//! context.
 
 use crate::entropy_common::FSE_readNCount;
 use crate::errors::{ERR_isError, ZstdErrorCode, ERROR};