[DOCS] Add severity field to case APIs (#132289)

docs/api/cases/cases-api-add-comment.asciidoc

@@ -120,6 +120,7 @@ The API returns details about the case and its comments. For example:
   },
   "owner": "cases",
   "duration": null,
+  "severity": "low",
   "closed_at": null,
   "closed_by": null,
   "created_at": "2022-03-24T00:37:03.906Z",

docs/api/cases/cases-api-create.asciidoc

@@ -140,6 +140,10 @@ An object that contains the case settings.
 (Required, boolean) Turns alert syncing on or off.
 ====
 
+`severity`::
+(Optional,string) The severity of the case. Valid values are: `critical`, `high`,
+`low`, and `medium`.
+
 `tags`::
 (Required, string array) The words and phrases that help
 categorize cases. It can be an empty array.
@@ -206,6 +210,7 @@ the case identifier, version, and creation time. For example:
   "owner": "securitySolution",
   "description": "James Bond clicked on a highly suspicious email banner advertising cheap holidays for underpaid civil servants. Operation bubblegum is active. Repeat - operation bubblegum is now active",
   "duration": null,
+  "severity": "low",
   "closed_at": null,
   "closed_by": null,
   "created_at": "2022-05-13T09:16:17.416Z",

docs/api/cases/cases-api-find-cases.asciidoc

@@ -62,6 +62,10 @@ filters the objects in the response.
 (Optional, string or array of strings) The fields to perform the
 `simple_query_string` parsed query against.
 
+`severity`::
+(Optional,string) The severity of the case. Valid values are: `critical`, `high`,
+`low`, and `medium`.
+
 `sortField`::
 (Optional, string) Determines which field is used to sort the results,
 `createdAt` or `updatedAt`. Defaults to `createdAt`.
@@ -126,6 +130,7 @@ The API returns a JSON object listing the retrieved cases. For example:
       },
       "owner": "securitySolution",
       "duration": null,
+      "severity": "low",
       "closed_at": null,
       "closed_by": null,
       "created_at": "2022-03-29T13:03:23.533Z",