Add documentation for contains(), union(), length(), and empty()

docs/reference/schemas/config/functions/contains.md

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+---
+description: Reference for the 'contains' DSC configuration document function
+ms.date:     08/08/2025
+ms.topic:    reference
+title:       contains
+---
+
+# contains
+
+## Synopsis
+
+Checks whether a collection contains a specific value or whether a string
+contains a substring.
+
+## Syntax
+
+```Syntax
+contains(<collection>, <value>)
+```
+
+## Description
+
+The `contains()` function checks whether a collection (array, object, or
+string) contains a specific value, returning `true` if it does and `false`
+otherwise. For arrays, it checks if the value exists as an element. For
+objects, it checks if the value exists as a property key or value. For
+strings, it checks if the value exists as a substring.
+
+The function accepts string and number values for the search parameter when
+used with arrays, objects, or strings.
+
+## Examples
+
+### Example 1 - Check array for values
+
+The following example shows how to check if an array contains specific values.
+
+```yaml
+# contains.example.1.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  myArray:
+    type: array
+    defaultValue:
+    - apple
+    - banana
+    - 42
+    - true
+resources:
+- name: Check array contents
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      hasApple:    "[contains(parameters('myArray'), 'apple')]"
+      hasOrange:   "[contains(parameters('myArray'), 'orange')]"
+      hasNumber42: "[contains(parameters('myArray'), 42)]"
+      hasNumber99: "[contains(parameters('myArray'), 99)]"
+```
+
+```bash
+dsc config get --file contains.example.1.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Check array contents
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        hasApple: true
+        hasOrange: false
+        hasNumber42: true
+        hasNumber99: false
+messages: []
+hadErrors: false
+```
+
+### Example 2 - Check object for keys and values
+
+The following example shows how to check if an object contains specific keys
+or values.
+
+```yaml
+# contains.example.2.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  myObject:
+    type: object
+    defaultValue:
+      name: John
+      age: 30
+      city: Seattle
+resources:
+- name: Check object contents
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      hasNameKey:      "[contains(parameters('myObject'), 'name')]"
+      hasEmailKey:     "[contains(parameters('myObject'), 'email')]"
+      hasSeattleValue: "[contains(parameters('myObject').city, 'Seattle')]"
+      hasAge30Value:   "[contains(parameters('myObject').age, 30)]"
+```
+
+```bash
+dsc config get --file contains.example.2.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Check object contents
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        hasNameKey: true
+        hasEmailKey: false
+        hasSeattleValue: true
+        hasAge30Value: true
+messages: []
+hadErrors: false
+```
+
+### Example 3 - Check string for substrings
+
+The following example shows how to check if a string contains specific
+substrings.
+
+```yaml
+# contains.example.3.dsc.config.yaml
+$schema: https://aka.ms/dsc/schemas/v3/bundled/config/document.json
+parameters:
+  myString:
+    type: string
+    defaultValue: "Hello DSC 123"
+resources:
+- name: Check string contents
+  type: Microsoft.DSC.Debug/Echo
+  properties:
+    output:
+      hasHello:  "[contains(parameters('myString'), 'Hello')]"
+      hasDSC:  "[contains(parameters('myString'), 'DSC')]"
+      hasNumber: "[contains(parameters('myString'), '123')]"
+      hasXYZ:    "[contains(parameters('myString'), 'XYZ')]"
+```
+
+```bash
+dsc config get --file contains.example.3.dsc.config.yaml
+```
+
+```yaml
+results:
+- name: Check string contents
+  type: Microsoft.DSC.Debug/Echo
+  result:
+    actualState:
+      output:
+        hasHello: true
+        hasDSC: true
+        hasNumber: true
+        hasXYZ: false
+messages: []
+hadErrors: false
+```
+
+## Parameters
+
+### collection
+
+The collection to search in (array, object, or string).
+
+```yaml
+Type:         [array, object, string]
+Required:     true
+```
+
+### value
+
+The value to search for. Must be a string or number.
+
+```yaml
+Type:         [string, number]
+Required:     true
+```
+
+The `contains()` function expects exactly two input values. The first
+parameter is the collection to search, and the second is the value to find.
+Complex objects and arrays cannot be used as search values.
+
+## Output
+
+The `contains()` function returns `true` if the collection contains the
+specified value and `false` otherwise.
+
+```yaml
+Type: bool
+```
+
+<!-- Link reference definitions -->