doc: mark Certificate methods as static, add missing KeyObject.from

doc/api/crypto.md

@@ -53,7 +53,7 @@ The `crypto` module provides the `Certificate` class for working with SPKAC
 data. The most common usage is handling output generated by the HTML5
 `<keygen>` element. Node.js uses [OpenSSL's SPKAC implementation][] internally.
 
-### `Certificate.exportChallenge(spkac[, encoding])`
+### Static method: `Certificate.exportChallenge(spkac[, encoding])`
 <!-- YAML
 added: v9.0.0
 changes:
@@ -76,7 +76,7 @@ console.log(challenge.toString('utf8'));
 // Prints: the challenge as a UTF8 string
 ```
 
-### `Certificate.exportPublicKey(spkac[, encoding])`
+### Static method: `Certificate.exportPublicKey(spkac[, encoding])`
 <!-- YAML
 added: v9.0.0
 changes:
@@ -99,7 +99,7 @@ console.log(publicKey);
 // Prints: the public key as <Buffer ...>
 ```
 
-### `Certificate.verifySpkac(spkac[, encoding])`
+### Static method: `Certificate.verifySpkac(spkac[, encoding])`
 <!-- YAML
 added: v9.0.0
 changes:
@@ -1284,6 +1284,32 @@ passing keys as strings or `Buffer`s due to improved security features.
 The receiver obtains a cloned `KeyObject`, and the `KeyObject` does not need to
 be listed in the `transferList` argument.
 
+### Static method: `KeyObject.from(key)`
+<!-- YAML
+added: v15.0.0
+-->
+
+* `key` {CryptoKey}
+* Returns: {KeyObject}
+
+Example: Converting a `CryptoKey` instance to a `KeyObject`:
+
+```js
+const { webcrypto: { subtle }, KeyObject } = require('crypto');
+
+(async function() {
+  const key = await subtle.generateKey({
+    name: 'HMAC',
+    hash: 'SHA-256',
+    length: 256
+  }, true, ['sign', 'verify']);
+
+  const keyObject = KeyObject.from(key);
+  console.log(keyObject.symmetricKeySize);
+  // Prints: 32 (symmetric key size in bytes)
+})();
+```
+
 ### `keyObject.asymmetricKeyDetails`
 <!-- YAML
 added: v15.7.0