Dev Docs: clarify null data conditions

_data/glossary/en/null-data-transaction.yaml

@@ -7,8 +7,10 @@ required:
     title_max_40_characters_no_formatting: Null Data (OP_RETURN) Transaction
 
     summary_max_255_characters_no_formatting: >
-        A standard transaction type which allows adding 80 bytes of
-        arbitrary data to the block chain up to once per transaction.
+        A transaction type relayed and mined by default in Bitcoin Core
+        0.9.0 and later that adds arbitrary data to a provably
+        unspendable pubkey script that full nodes don't have to store in
+        their UTXO database.
 
     synonyms_shown_in_glossary_capitalize_first_letter:
         - Null data transaction