storage: clarify documentation about download integrity

https://github.com/GoogleCloudPlatform/gcloud-node/pull/310\#discussion-diff-21434792

Author: stephenplusplus

lib/storage/file.js

@@ -219,9 +219,10 @@ File.prototype.copy = function(destination, callback) {
  * @param {object=} options - Configuration object.
  * @param {string|boolean} options.validation - Possible values: `"md5"`,
  *     `"crc32c"`, or `false`. By default, data integrity is validated with an
- *     MD5 checksum for maximum reliability. CRC32c will provide better
- *     performance with less reliability. You may also choose to skip validation
- *     completely, however this is **not recommended**.
+ *     MD5 checksum for maximum reliability, falling back to CRC32c when an MD5
+ *     hash wasn't returned from the API. CRC32c will provide better performance
+ *     with less reliability. You may also choose to skip validation completely,
+ *     however this is **not recommended**.
  *
  * @example
  * //-