Update Get-AdvancedThreatProtectionDocumentReport.md

Added Description for SummarizeBy param per Issue #8292.

Author: chrisda

exchange/exchange-ps/exchange/Get-AdvancedThreatProtectionDocumentReport.md

@@ -221,7 +221,16 @@ Accept wildcard characters: False
 ```
 
 ### -SummarizeBy
-{{ Fill SummarizeBy Description }}
+The SummarizeBy parameter returns totals based on the values you specify. If your report filters data using any of the values accepted by this parameter, you can use the SummarizeBy parameter to summarize the results based on those values. To decrease the number of rows returned in the report, consider using the SummarizeBy parameter. Summarizing reduces the amount of data that's retrieved for the report, and delivers the report faster. For example, instead of seeing each instance of a specific value of EventType on an individual row in the report, you can use the SummarizeBy parameter to see the total number of instances of that value of EventType on one row in the report.
+
+For this cmdlet, valid values are:
+
+- Action
+- Direction
+- Domain
+- EventType
+
+You can specify multiple values separated by commas.
 
 ```yaml
 Type: MultiValuedProperty