add better explination why frozen-abi re-exports rand, rand_chacha and bincode

Author: puhtaytow

Cargo.lock

@@ -37,7 +37,10 @@ thiserror = { workspace = true }
 [target.'cfg(not(target_os = "solana"))'.dependencies]
 im = { workspace = true, features = ["rayon", "serde"] }
 memmap2 = { workspace = true }
-# these are re-exports for StableAbi
+
+# Canonical versions re-exported for `StableAbi`.
+# These dependencies are used only to back `frozen-abi` StableAbi API,
+# to avoid version skew and keep digest computation stable regardless of consumer dependencies.
 rand = "0.8.5"
 rand_chacha = "0.3.1"
 bincode = "1.3.3"