doc,crypto: add null length to crypto.subtle.deriveBits

doc/api/webcrypto.md

@@ -575,16 +575,24 @@ changes:
 
 * `algorithm`: {AlgorithmIdentifier|EcdhKeyDeriveParams|HkdfParams|Pbkdf2Params}
 * `baseKey`: {CryptoKey}
-* `length`: {number}
+* `length`: {number|null}
 * Returns: {Promise} containing {ArrayBuffer}
 
 <!--lint enable maximum-line-length remark-lint-->
 
 Using the method and parameters specified in `algorithm` and the keying
 material provided by `baseKey`, `subtle.deriveBits()` attempts to generate
-`length` bits. The Node.js implementation requires that `length` is a
-multiple of `8`. If successful, the returned promise will be resolved with
-an {ArrayBuffer} containing the generated data.
+`length` bits.
+
+The Node.js implementation requires that when `length` is a
+number it must be multiple of `8`.
+
+When `length` is `null` the maximum number
+of bits for a given algorithm is generated. This is allowed for the
+`'ECDH'`, `'X25519'`[^1], and `'X448'`[^1] algorithms.
+
+If successful, the returned promise will
+be resolved with an {ArrayBuffer} containing the generated data.
 
 The algorithms currently supported include: