Docs: update annotations API docs to include required field (#52644)

* Update annotations.md

A minor change to include the required fields when creating a new annotation via the annotations API. The error message when fields are missing is not descriptive and there is no indication in the current documentation (that I can see anyway) that would indicate these fields are required.

* Update annotations.md

Update required json body fields to only include fields that are actually required.

* Add required field in Swagger

Co-authored-by: Sofia Papagiannaki <1632407+papagian@users.noreply.github.com>

pkg/api/dtos/annotations.go

@@ -3,14 +3,15 @@ package dtos
 import "github.com/grafana/grafana/pkg/components/simplejson"
 
 type PostAnnotationsCmd struct {
-	DashboardId  int64            `json:"dashboardId"`
-	DashboardUID string           `json:"dashboardUID,omitempty"`
-	PanelId      int64            `json:"panelId"`
-	Time         int64            `json:"time"`
-	TimeEnd      int64            `json:"timeEnd,omitempty"` // Optional
-	Text         string           `json:"text"`
-	Tags         []string         `json:"tags"`
-	Data         *simplejson.Json `json:"data"`
+	DashboardId  int64  `json:"dashboardId"`
+	DashboardUID string `json:"dashboardUID,omitempty"`
+	PanelId      int64  `json:"panelId"`
+	Time         int64  `json:"time"`
+	TimeEnd      int64  `json:"timeEnd,omitempty"` // Optional
+	// required: true
+	Text string           `json:"text"`
+	Tags []string         `json:"tags"`
+	Data *simplejson.Json `json:"data"`
 }
 
 type UpdateAnnotationsCmd struct {