[Synthetics] Add namespace option docs (#3824)

### Summary

Add missing namespace option docs added on
elastic/synthetics#1021.

---------

Co-authored-by: florent-leborgne <[email protected]>

Author: emilioalvap

solutions/observability/synthetics/configure-lightweight-monitors.md

@@ -364,6 +364,19 @@ $$$monitor-fields$$$
     fields.team: synthetics
     ```
 
+$$$monitor-namespace$$$
+
+**`namespace`**
+:   Type: [string](/solutions/observability/synthetics/configure-lightweight-monitors.md#synthetics-lightweight-data-string)  
+
+    Assign the monitor to a specific datastream namespace. Defaults to Kibana's namespace.
+
+    **Examples**:
+
+    ```yaml
+    namespace: custom-namespace
+    ```
+
 ### HTTP options [synthetics-lightweight-http]
 
 The options described here configure Synthetics to connect via HTTP and optionally verify that the host returns the expected response.

solutions/observability/synthetics/configure-projects.md

@@ -281,6 +281,15 @@ $$$synthetics-configuration-monitor-tags$$$ `tags` (`Array<string>`)
     }
     ```
 
+`namespace` (`string`)
+:   Assign the monitor to a specific datastream namespace. Defaults to Kibana's namespace.
+
+    For example:
+
+    ```js
+    namespace: 'custom-namespace'
+    ```
+
 For information on configuring monitors individually, refer to:
 
 * [Configure individual browser monitors](/solutions/observability/synthetics/configure-individual-browser-monitors.md) for browser monitors