Clarify note on */CLIENTSIDEASSETS origin requirement (SharePoint#1541)

Added clarification inside of a note block on requirement of */CLIENTSIDEASSETS origin. If this origin is not present and the public CDN is enabled, requests for SPFx bundles will contain the token spclientsideassetlibrary. 

This could potentially be a bug, as it is feasible for tenants to desire to use the CDN functionality in a limited capacity (say for a central /CDN origin) and disable the out-of-box origins to reduce overall exposure.

Author: jonthenerd

docs/spfx/web-parts/get-started/hosting-webpart-from-office-365-cdn.md

@@ -67,7 +67,7 @@ You can also follow these steps by watching this video on the [SharePoint PnP Yo
   SharePoint Framework solutions can automatically benefit from the Office 365 Public CDN as long as it's enabled in your tenant. When CDN is enabled, the `*/CLIENTSIDEASSETS` origin is automatically added as a valid origin.
 
   > [!NOTE]
-  > If you have previously enabled Office 365 CDN, you should re-enable the public CDN so that you have the `*/CLIENTSIDEASSETS`entry added as a valid CDN origin for public CDN.
+  > If you have previously enabled Office 365 CDN, you should re-enable the public CDN so that you have the `*/CLIENTSIDEASSETS`entry added as a valid CDN origin for public CDN. If this entry is not present and the public CDN is enabled in your tenant, bundle requests will contain the token hostname `spclientsideassetlibrary` in their URL, causing the requests to fail.
 
 6. You can double-check the current setup of your end-points. Execute the following command to get the list of CDN origins from your tenant: