Add shutdown API reference page [doc only]

Author: brizental

docs/user/SUMMARY.md

@@ -38,6 +38,7 @@
     - [Toggling upload status](reference/general/toggling-upload-status.md)
     - [Annotating experiments](reference/general/experiments-api.md)
     - [Registering custom pings](reference/general/register-custom-pings.md)
+    - [Shut down](reference/general/shutdown.md)
 - [Debugging](reference/debug/index.md)
     - [Log pings](reference/debug/logPings.md)
     - [Debug View Tag](reference/debug/debugViewTag.md)

docs/user/reference/general/shutdown.md

@@ -0,0 +1,60 @@
+# Shut down
+
+Provides a way for users to gracefully shut down Glean,
+by blocking until it is finished performing pending tasks,
+such as recording metrics and uploading pings.
+
+{{#include ../../../shared/blockquote-info.html}}
+
+## How the Glean SDKs execute tasks
+
+> Most calls to Glean APIs are dispatched[^1]. This strategy is adopted, because most tasks
+> performed by the Glean SDKs involve file system read or write operations, HTTP requests and
+> other time consuming actions.
+>
+> Each Glean SDK has an internal structure called "Dispatcher" which makes sure API calls
+> get executed in the order they were called, while not requiring the caller to block on the
+> completion of each of these tasks.
+>
+> [^1]: Here this term indicates the tasks are run asynchronously in JavaScript or in a different
+> thread for all other SDKs.
+
+## API
+
+### `shutdown`
+
+{{#include ../../../shared/tab_header.md}}
+<div data-lang="Kotlin" class="tab"></div>
+<div data-lang="Java" class="tab"></div>
+<div data-lang="Swift" class="tab"></div>
+<div data-lang="Python" class="tab"></div>
+<div data-lang="Rust" class="tab"></div>
+<div data-lang="JavaScript" class="tab">
+
+```js
+import Glean from "@mozilla/glean/webext";
+
+async function onUninstall() {
+  // Flips Glean upload status to `false`,
+  // which triggers sending of a `deletion-request` ping.
+  Glean.setUploadEnabled(false);
+
+  // Block on shut down to guarantee all pending pings
+  // (including the `deletion-request` sent above)
+  // are sent before the extension is uninstalled.
+  await Glean.shutdown();
+
+  // Uninstall browser extension without asking for user approval before doing so.
+  await browser.management.uninstallSelf({ showConfirmDialog: false });
+}
+```
+
+The `shutdown` API is available for all JavaScript targets, even though the above example
+is explicitly using the `webext` target.
+</div>
+<div data-lang="Firefox Desktop" class="tab"></div>
+{{#include ../../../shared/tab_footer.md}}
+
+## Reference
+
+* [JavaScript API docs](https://mozilla.github.io/glean.js/classes/core_glean.default.html#shutdown)