Update append array docs

pkg/yqlib/doc/Add.md

@@ -127,6 +127,32 @@ b:
   - 4
 ```
 
+## Append another array using +=
+Given a sample.yml file of:
+```yaml
+a:
+  - 1
+  - 2
+b:
+  - 3
+  - 4
+```
+then
+```bash
+yq eval '.a += .b' sample.yml
+```
+will output
+```yaml
+a:
+  - 1
+  - 2
+  - 3
+  - 4
+b:
+  - 3
+  - 4
+```
+
 ## Relative append
 Given a sample.yml file of:
 ```yaml

pkg/yqlib/operator_add_test.go

@@ -14,14 +14,6 @@ var addOperatorScenarios = []expressionScenario{
 			"D0, P[1 a], (!!int)::3\n",
 		},
 	},
-	{
-		skipDoc:    true,
-		document:   "array: [1]\narray2: [2]",
-		expression: ".array += .array2",
-		expected: []string{
-			"D0, P[], (doc)::array: [1, 2]\narray2: [2]\n",
-		},
-	},
 	{
 		skipDoc:    true,
 		document:   `{}`,
@@ -94,6 +86,14 @@ var addOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::{a: [1, 2, 3, 4], b: [3, 4]}\n",
 		},
 	},
+	{
+		description: "Append another array using +=",
+		document:    `{a: [1,2], b: [3,4]}`,
+		expression:  `.a += .b`,
+		expected: []string{
+			"D0, P[], (doc)::{a: [1, 2, 3, 4], b: [3, 4]}\n",
+		},
+	},
 	{
 		description: "Relative append",
 		document:    `a: { a1: {b: [cat]}, a2: {b: [dog]}, a3: {} }`,