docs: document tls/crypto `ciphers` option

Hitherto undocumented option that lets the user select the list of ciphers to
use or exclude in a SSL/TLS session.

doc/api/crypto.markdown

@@ -15,6 +15,9 @@ Creates a credentials object, with the optional details being a dictionary with
 * `key` : a string holding the PEM encoded private key
 * `cert` : a string holding the PEM encoded certificate
 * `ca` : either a string or list of strings of PEM encoded CA certificates to trust.
+* `ciphers`: a string describing the ciphers to use or exclude. Consult
+  <http://www.openssl.org/docs/apps/ciphers.html#CIPHER_LIST_FORMAT> for details
+  on the format.
 
 If no 'ca' details are given, then node.js will use the default publicly trusted list of CAs as given in
 <http://mxr.mozilla.org/mozilla/source/security/nss/lib/ckfw/builtins/certdata.txt>.

doc/api/tls.markdown

@@ -46,6 +46,10 @@ The `options` object has these possibilities:
     omitted several well known "root" CAs will be used, like VeriSign.
     These are used to authorize connections.
 
+  - `ciphers`: A string describing the ciphers to use or exclude. Consult
+    <http://www.openssl.org/docs/apps/ciphers.html#CIPHER_LIST_FORMAT> for
+    details on the format.
+
   - `requestCert`: If `true` the server will request a certificate from
     clients that connect and attempt to verify that certificate. Default:
     `false`.