Explain new Mixpanel Functionality

Mixpanel released new features around aliasing anonymous users into known users: https://help.mixpanel.com/hc/en-us/articles/360039133851#enable-id-merge, updating documents to reflect this information as it changes segment capabilities as outlined here: https://segment.slack.com/archives/CB01T2DN0/p1584613766018300

Author: segment-sv

src/connections/destinations/catalog/mixpanel/index.md

@@ -190,7 +190,7 @@ If you're not familiar with the Segment Specs, take a look to understand what th
 ```javascript
 analytics.alias('newUserId')
 ```
-**Important:** Calling `alias` is required for Mixpanel in all libraries in order to connect anonymous visitors to identified users.
+**Important:** Calling `alias` used to be required for Mixpanel in all libraries in order to connect anonymous visitors to identified users. However, with the release of Mixpanel's New [identity merge feature](https://help.mixpanel.com/hc/en-us/articles/360039133851#enable-id-merge) this is no longer necessary if you enable the ID Merge Setting under Mixpanel -> Project Settings -> Identity Merge. **If you aren't using this setting, please continue with the instructions below.**
 
 As soon as you have a `userId` for a visitor that was previously anonymous you'll need to [`alias`](/docs/connections/spec/alias/) their old anonymous `id` to the new `userId`. In Mixpanel only **one** anonymous user history can be merged to **one** identified user. For that reason you should only call `alias` once, right after a user registered, but before the first `identify`.