doc: add example using algorithms not directly exposed

PR-URL: https://github.com/nodejs/node/pull/6108
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>

doc/api/crypto.markdown

@@ -734,6 +734,28 @@ console.log(sign.sign(private_key, 'hex'));
   // Prints the calculated signature
 ```
 
+A [`sign`][] instance can also be created by just passing in the digest
+algorithm name, in which case OpenSSL will infer the full signature algorithm
+from the type of the PEM-formatted private key, including algorithms that
+do not have directly exposed name constants, e.g. 'ecdsa-with-SHA256'.
+
+Example: signing using ECDSA with SHA256
+
+```js
+const crypto = require('crypto');
+const sign = crypto.createSign('sha256');
+
+sign.update('some data to sign');
+
+const private_key = '-----BEGIN EC PRIVATE KEY-----\n' +
+        'MHcCAQEEIF+jnWY1D5kbVYDNvxxo/Y+ku2uJPDwS0r/VuPZQrjjVoAoGCCqGSM49\n' +
+        'AwEHoUQDQgAEurOxfSxmqIRYzJVagdZfMMSjRNNhB8i3mXyIMq704m2m52FdfKZ2\n' +
+        'pQhByd5eyj3lgZ7m7jbchtdgyOF8Io/1ng==\n' +
+        '-----END EC PRIVATE KEY-----\n';
+
+console.log(sign.sign(private_key).toString('hex'));
+```
+
 ### sign.sign(private_key[, output_format])
 
 Calculates the signature on all the data passed through using either