Clarify manage_data value in SEP-10 (#366)

ecosystem/sep-0010.md

@@ -81,7 +81,7 @@ On success the endpoint must return `200 OK` HTTP status code and a JSON object
   * time bounds: `{min: now(), max: now() + 300 }` (we recommend expiration of 5 minutes to give user time to sign transaction)
   * operations: `manage_data(source: client_account, key: '<anchor name> auth', value: random_nonce())`
     * The value of key is not important, but can be the name of the anchor followed by `auth`. It can be at most 64 bytes.
-    * The value must be a 64 byte long base64 encoded cryptographic-quality random string
+    * The value must be 64 bytes long. It contains a 48 byte cryptographic-quality random string encoded using base64 (for a total of 64 bytes after encoding).
   * signature by the web service signing account
 * `network_passphrase`: (optional but recommended) Stellar network passphrase used by the server. This allows the client to verify that it's using the correct passphrase when signing.