Tweak cipherparams description, add getDefaultParams

content/client-lib-development-guide/features.textile

@@ -557,7 +557,10 @@ h4. ChannelOptions
 
 h4. CipherParams
 * @(TZ1)@ params to configure encryption for a channel, see "Java CipherParams class":https://github.com/ably/ably-java/blob/master/src/io/ably/util/Crypto.java#L52 as a reference
-* @(TZ2)@ The attributes of @CipherParams@ consist of:
+* @(TZ2)@ The attributes of @CipherParams@ consist of anything necessary to implement the supported algorithms (eg key and iv), in addition to the following standardised attributes:
 ** @(TZ2a)@ @algorithm@ string - Default is @AES@. Optionally specify the algorithm to use for encryption, currently only @AES@ is supported
-** @(TZ2b)@ @keyLength@ integer - Default is 128. Optionally specify the key length of the cipher, typically 128 or 256
+** @(TZ2b)@ @keyLength@ integer - An integer, from an algorithm-dependent range of possible values. If unspecified, an algorithm-dependent default is used. For AES the allowed values at least include 128 and 256, with a default of 128.
 ** @(TZ2c)@ @mode@ string - Default is @CBC@. Optionally specify cipher mode, currently only @CBC@ is supported
+* @(TZ3)@ @Crypto#getDefaultParams@ function
+** @(TZ3a)@ returns a complete @CipherParams@ instance with randomly generated key and iv
+** @(TZ3b)@ takes an optional key parameter, returning a @CipherParams@ instance created with that key and with a keyLength calculated from that key