docs: add relevant Refer/Say/ssml links to upgrade guide; formatting (#895)

Updates the UPGRADE.md doc to provide docs links to relevant resources for changes that could be confusing to customers. Fixes a few markdownlint warnings.

Author: stern-shawn

UPGRADE.md

@@ -2,61 +2,75 @@
 
 _All `MAJOR` version bumps will have upgrade notes posted here._
 
-[2023-01-25] 3.x.x to 4.x.x
------------------------------
+## [2023-01-25] 3.x.x to 4.x.x
+
+---
 
 * Supported Node.js versions updated
   * Upgrade to Node.js >= 14
   * Dropped support for Node.js < 14 ([#791](https://github.com/twilio/twilio-node/pull/791))
   * Added support for Node.js 18 ([#794](https://github.com/twilio/twilio-node/pull/794))
 * Lazy loading enabled by default ([#752](https://github.com/twilio/twilio-node/pull/752))
   * Required Twilio modules now lazy load by default
-  * See the [README](README.md) for how to disable lazy loading
+  * See the [README](README.md#lazy-loading) for how to disable lazy loading
 * Type changes from `object` to `Record` ([#873](https://github.com/twilio/twilio-node/pull/873))
   * Certain response properties now use the `Record` type with `string` keys
   * Including the `subresourceUris` property for v2010 APIs and the `links` properties for non-v2010 APIs
-* Creating an AccessToken requires an `identity` in the options ([#875](https://github.com/twilio/twilio-node/pull/875))
-* JWT token building deprecations
+* Access Tokens
+  * Creating an [AccessToken](https://www.twilio.com/docs/iam/access-tokens) requires an `identity` in the options ([#875](https://github.com/twilio/twilio-node/pull/875))
   * `ConversationsGrant` has been deprecated in favor of `VoiceGrant` ([#783](https://github.com/twilio/twilio-node/pull/783))
   * `IpMessagingGrant` has been removed ([#784](https://github.com/twilio/twilio-node/pull/784))
 * TwiML function deprecations ([#788](https://github.com/twilio/twilio-node/pull/788))
-  * `Refer.referSip()` replaced by `Refer.sip()`
-  * `ssmlBreak()` replaced by `break_()`
-  * `ssmlEmphasis()` replaced by `emphasis()`
-  * `ssmlLang()` replaced by `lang()`
-  * `ssmlP()` replaced by `p()`
-  * `ssmlPhoneme()` replaced by `phoneme()`
-  * `ssmlProsody()` replaced by `prosody()`
-  * `ssmlS()` replaced by `s()`
-  * `ssmlSayAs()` replaced by `sayAs()`
-  * `ssmlSub()` replaced by `sub()`
-  * `ssmlW()` replaced by `w()`
-    * Old:
-    ```js
-    const response = new VoiceResponse();
-    const say = response.say("Hello");
-    say.ssmlEmphasis("you");
-    ```
-    * New:
-    ```js
-    const response = new VoiceResponse();
-    const say = response.say("Hello");
-    say.emphasis("you");
-    ```
-* TaskRouter Workers Statistics operations updated ([#820](https://github.com/twilio/twilio-node/pull/820))
+  * [`<Refer>`](https://www.twilio.com/docs/voice/twiml/refer)
+    * `Refer.referSip()` replaced by `Refer.sip()`
+  * [`<Say>`](https://www.twilio.com/docs/voice/twiml/say/text-speech#generating-ssml-via-helper-libraries)
+    * `Say.ssmlBreak()` replaced by `Say.break()`
+    * `Say.ssmlEmphasis()` replaced by `Say.emphasis()`
+    * `Say.ssmlLang()` replaced by `Say.lang()`
+    * `Say.ssmlP()` replaced by `Say.p()`
+    * `Say.ssmlPhoneme()` replaced by `Say.phoneme()`
+    * `Say.ssmlProsody()` replaced by `Say.prosody()`
+    * `Say.ssmlS()` replaced by `Say.s()`
+    * `Say.ssmlSayAs()` replaced by `Say.sayAs()`
+    * `Say.ssmlSub()` replaced by `Say.sub()`
+    * `Say.ssmlW()` replaced by `Say.w()`
+
+      Old:
+
+      ```js
+      const response = new VoiceResponse();
+      const say = response.say("Hello");
+      say.ssmlEmphasis("you");
+      ```
+
+      New:
+
+      ```js
+      const response = new VoiceResponse();
+      const say = response.say("Hello");
+      say.emphasis("you");
+      ```
+
+* [TaskRouter Workers Statistics](https://www.twilio.com/docs/taskrouter/api/worker/statistics) operations updated ([#820](https://github.com/twilio/twilio-node/pull/820))
   * Cumulative and Real-Time Workers Statistics no longer accept a WorkerSid
   * `GET /v1/Workspaces/{WorkspaceSid}/Workers/CumulativeStatistics`
-    * Old: `client.taskrouter.v1.workspaces('WS...').workers('WK...).cumulativeStatistics()`
-    * New: `client.taskrouter.v1.workspaces('WS...').workers.cumulativeStatistics()`
+
+    Old: `client.taskrouter.v1.workspaces('WS...').workers('WK...).cumulativeStatistics()`
+
+    New: `client.taskrouter.v1.workspaces('WS...').workers.cumulativeStatistics()`
   * `GET /v1/Workspaces/{WorkspaceSid}/Workers/RealTimeStatistics`
-    * Old: `client.taskrouter.v1.workspaces('WS...').workers('WK...).realTimeStatistics()`
-    * New: `client.taskrouter.v1.workspaces('WS...').workers.realTimeStatistics()`
 
-[2017-05-22] 3.1.x to 3.2.x
----------------------------
+    Old: `client.taskrouter.v1.workspaces('WS...').workers('WK...).realTimeStatistics()`
+
+    New: `client.taskrouter.v1.workspaces('WS...').workers.realTimeStatistics()`
+
+## [2017-05-22] 3.1.x to 3.2.x
+
+---
 
 ### CHANGED - Rename video room `Recordings` class to `RoomRecordings`
 
 #### Rationale
-- This was done to avoid a class name conflict with another resource.
-- Client code should be unaffected unless you manipulate the Recording/RoomRecordings class directly. Accessing room recording metadata via the client should work the same way as before.
+
+* This was done to avoid a class name conflict with another resource.
+* Client code should be unaffected unless you manipulate the Recording/RoomRecordings class directly. Accessing room recording metadata via the client should work the same way as before.