Merge pull request #88230 from ishansrivastava90/docs-limit-per-shard

Adding info about docs limit per shard enforced by lucene.

Author: PRMerger8

articles/search/search-limits-quotas-capacity.md

@@ -56,12 +56,14 @@ Maximum limits on storage, workloads, and quantities of indexes, documents, and
 
 ## Document limits 
 
-As of October 2018, there are no longer any document limits for any new service created at any billable tier (Basic, S1, S2, S3, S3 HD) in any region. While most regions have had unlimited document counts since November/December 2017, there were five regions that continued to impose document limits. Depending on when and where you created a search service, you might be running a service that is still subject to document limits.
+As of October 2018, there are no longer any document limits<sup>1</sup> for any new service created at any billable tier (Basic, S1, S2, S3, S3 HD) in any region. While most regions have had unlimited document counts since November/December 2017, there were five regions that continued to impose document limits. Depending on when and where you created a search service, you might be running a service that is still subject to document limits.
 
 To determine whether your service has document limits, check the Usage tile in the overview page of your service. Document counts are either unlimited, or subject to a limit based on tier.
 
   ![Usage tile](media/search-limits-quotas-capacity/portal-usage-tile.png)
 
+<sup>1</sup> Even though there aren't any SKU specific document limits, every index is still subject to a maximum safe limit to ensure stability of the service. This limit comes from Lucene. Every Azure Search document is internally indexed as one or more Lucene documents. The number of Lucene documents per Azure search document depends on the total number of elements in complex collection fields. Each element is indexed as a separate Lucene document. For example, a document with 3 elements in a complex collection field, will be indexed as 4 Lucene documents - 1 for the document itself and 3 for the elements. The maximum number of Lucene documents is roughly 25 billion per index.
+
 ### Regions previously having document limits
 
 If the portal indicates a document limit, your service was either created before late 2017, or it was created on a data center using lower-capacity clusters for hosting Azure Search services: