Merge pull request #179214 from mattmsft/magottei/update-sharepoint-docs

Update SharePoint Online indexer documentation

Author: PRMerger6

articles/search/search-howto-index-sharepoint-online.md

@@ -150,6 +150,9 @@ api-key: [admin key]
 
 ```
 
+> [!IMPORTANT]
+> Only [`metadata_spo_site_library_item_id`](#metadata) may be used as the key field in an index populated by the SharePoint Online indexer. If a key field doesn't exist in the data source, `metadata_spo_site_library_item_id` is automatically mapped to the key field.
+
 For more information, see [Create Index (REST API)](/rest/api/searchservice/create-index).
 
 ### Step 5: Create an indexer
@@ -260,6 +263,8 @@ Once the data source has been updated, follow the below steps:
 
 1.	Manually kick off an indexer run again and check the indexer status. This time the indexer run should successfully start.
 
+<a name="metadata"></a>
+
 ## Indexing document metadata
 If you have set the indexer to index document metadata, the following metadata will be available to index.