Update overview-of-search.md

Adding information on identifier modifier not supported by AHDS and Azure API for FHIR

Author: EXPEkesheth

articles/healthcare-apis/fhir/overview-of-search.md

@@ -118,6 +118,7 @@ For more information, see the HL7 [Composite Search Parameters](https://www.hl7.
 |  `:below` (token) | No                 | No                        |
 |  `:above` (token) | No                 | No                        |
 |  `:not-in` (token) | No                | No                        |
+|  `:identifier` |No                     | No                        |
 
 For search parameters that have a specific order (numbers, dates, and quantities), you can use a [prefix](https://www.hl7.org/fhir/search.html#prefix) before the parameter value to refine the search criteria (for example, `Patient?_lastUpdated=gt2022-08-01` where the prefix `gt` means "greater than"). The FHIR service in Azure Health Data Services supports all prefixes defined in the FHIR standard.