Document why blockhash() is not available on Solana
@@ -2869,10 +2869,18 @@ block, or any block except for the most recent 256. Do not use this a source of
randomness unless you know what you are doing.
.. note::
-
This function is not available on Parity Substrate. When using Parity Substrate,
use ``random()`` as a source of random data.
+.. note::
+ This function is not available on Solana. There is the
+ `recent block hashes account <https://edge.docs.solana.com/developing/runtime-facilities/sysvars#recentblockhashes>`_
+ that looks useful at first glance, however it is not usable because:
+
+ - This account is `deprecated <https://github.com/solana-labs/solana/pull/18875>`_.
+ - It does not give any slot of block number, so it is not possible to provide a matching
+ function signature.
+
random(bytes subject) returns (bytes32)
+++++++++++++++++++++++++++++++++++++++
