doc: fix crypto update() signatures

PR-URL: https://github.com/nodejs/node/pull/5500
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>

doc/api/crypto.markdown

@@ -669,10 +669,15 @@ provided a string is returned; otherwise a [`Buffer`][] is returned;
 The `Hmac` object can not be used again after `hmac.digest()` has been
 called. Multiple calls to `hmac.digest()` will result in an error being thrown.
 
-### hmac.update(data)
+### hmac.update(data[, input_encoding])
 
-Update the `Hmac` content with the given `data`. This can be called
+Updates the `Hmac` content with the given `data`, the encoding of which
-many times with new data as it is streamed.
+is given in `input_encoding` and can be `'utf8'`, `'ascii'` or
+`'binary'`. If `encoding` is not provided, and the `data` is a string, an
+encoding of `'utf8'` is enforced. If `data` is a [`Buffer`][] then
+`input_encoding` is ignored.
+
+This can be called many times with new data as it is streamed.
 
 ## Class: Sign
 
@@ -733,10 +738,15 @@ returned.
 The `Sign` object can not be again used after `sign.sign()` method has been
 called. Multiple calls to `sign.sign()` will result in an error being thrown.
 
-### sign.update(data)
+### sign.update(data[, input_encoding])
 
-Updates the sign object with the given `data`. This can be called many times
+Updates the `Sign` content with the given `data`, the encoding of which
-with new data as it is streamed.
+is given in `input_encoding` and can be `'utf8'`, `'ascii'` or
+`'binary'`. If `encoding` is not provided, and the `data` is a string, an
+encoding of `'utf8'` is enforced. If `data` is a [`Buffer`][] then
+`input_encoding` is ignored.
+
+This can be called many times with new data as it is streamed.
 
 ## Class: Verify
 
@@ -780,10 +790,15 @@ console.log(verify.verify(public_key, signature));
   // Prints true or false
 ```
 
-### verifier.update(data)
+### verifier.update(data[, input_encoding])
 
-Updates the verifier object with the given `data`. This can be called many
+Updates the `Verify` content with the given `data`, the encoding of which
-times with new data as it is streamed.
+is given in `input_encoding` and can be `'utf8'`, `'ascii'` or
+`'binary'`. If `encoding` is not provided, and the `data` is a string, an
+encoding of `'utf8'` is enforced. If `data` is a [`Buffer`][] then
+`input_encoding` is ignored.
+
+This can be called many times with new data as it is streamed.
 
 ### verifier.verify(object, signature[, signature_format])