feat: add note about transferability

docs/concepts/protocol/04-nft.mdx

@@ -19,6 +19,11 @@ The transferability of the NFT makes Sablier streams tradable and usable as coll
 marketplace that allows users to borrow funds by locking their streams as collateral (effectively borrowing against
 their future income). Or a decentralized exchange that allows users to trade streams for other assets.
 
+:::note
+
+Not all Sablier streams are transferable. The stream creator can choose to make the stream
+[non-transferable](/contracts/v2/guides/stream-management/transfer).
+
 :::tip
 
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