Add detail about purpose, which breaks the 'not storing transparent addresses' rule

zip-0400.rst

@@ -74,8 +74,9 @@ than transparent keys.
 
 The reader should note that Zcash addresses are almost never stored in a
 wallet.dat file. Only transparent public keys and private keys exist in
-wallet.dat files, transparent addresses never appear or are stored in any way,
-they are calculated from pubkeys by nodes when the wallet is loaded.
+wallet.dat files, transparent addresses in general, are not stored directly,
+they are calculated from pubkeys by nodes when the wallet is loaded. Metadata
+about some transparent addresses is stored, such as in the `purpose` key type.
 
 Sprout shielded addresses work the same as above, only pubkeys and private keys
 are stored in wallet.dat. But in Zcash Sapling, there can be more than one
@@ -238,7 +239,8 @@ purpose
 
 Purpose of an address, i.e. "receive" or "change" etc.
 
-  * Size: 8 bytes
+  * Key Size: 36 bytes
+  * Value Size: 8 bytes
 
 sapzaddr
 =========