Add explanation of "keys" field. (hashicorp#9676)

website/pages/api-docs/secret/transit/index.mdx

@@ -133,6 +133,12 @@ $ curl \
 }
 ```
 
+The `keys` attribute lists each version of the key, and the time that key was created as seconds since the Unix epoch.
+The sample response shows a key that was created on September 22, 2015 7:50:12 PM GMT, and has not been rotated.
+
+The fields `supports_encryption`, `supports_decryption`, `supports_derivation` and `supports_signing` are
+derived from the type of the key, and indicate which operations may be performed with it.
+
 ## List Keys
 
 This endpoint returns a list of keys. Only the key names are returned (not the