Document acting-user updates to custom post text fallback

docs/capabilities/server/text_fallback.mdx

@@ -85,13 +85,58 @@ const post = await reddit.submitCustomPost({
 
 ## Update a post’s text fallback
 
-The post author can edit and update text fallback content after it’s been created. To do this, call post.setTextFallback with the desired fallback content.
+Call `post.setTextFallback()` to update the fallback content of an existing custom post. The method uses the app identity by default. For an app-authored post, omit `runAs` or set it to `"APP"`:
 
 ```tsx
-import { reddit } from '@devvit/web/server';
+import { context, reddit } from "@devvit/web/server";
+
+export async function updateAppFallback() {
+  if (!context.postId) {
+    throw new Error("Post context required.");
+  }
+
+  const post = await reddit.getPostById(context.postId);
+  await post.setTextFallback({ text: "Updated instructions for this post." });
+}
+```
+
+### Update a user-authored post
+
+Starting with Devvit 0.14.3, you can pass `runAs: "USER"` to update fallback content on behalf of the current user. Enable the `SUBMIT_POST` scope:
+
+```json title="devvit.json"
+{
+  "permissions": {
+    "reddit": {
+      "enable": true,
+      "asUser": ["SUBMIT_POST"]
+    }
+  }
+}
+```
+
+Call this function from a signed-in post interaction. This example checks that the current user is the post author before requesting an update:
+
+```tsx
+import { context, reddit } from "@devvit/web/server";
+
+export async function updateUserFallback() {
+  if (!context.userId || !context.postId) {
+    throw new Error("Signed-in post interaction required.");
+  }
+
+  const post = await reddit.getPostById(context.postId);
+  if (post.authorId !== context.userId) {
+    throw new Error("Post author required.");
+  }
 
-// from a menu action, form, scheduler, trigger, custom post click event, etc
-const newTextFallback = { text: 'This is an updated text fallback' };
-const post = await reddit.getPostById(context.postId);
-await post.setTextFallback(newTextFallback);
+  await post.setTextFallback({
+    text: "Updated instructions for this interactive post.",
+    runAs: "USER",
+  });
+}
 ```
+
+`"USER"` uses the current request's user. It cannot select an arbitrary user or bypass Reddit's edit permissions. User grants and authorization still apply, and a scheduler or trigger handler may not have a suitable acting-user context.
+
+The method returns `Promise<void>` and rejects if the update fails. Await it before reporting success. See [SetCustomPostTextFallbackOptions](../../api/redditapi/models/type-aliases/SetCustomPostTextFallbackOptions.md) for the content options and `runAs` type.

versioned_docs/version-0.14/capabilities/server/text_fallback.mdx

@@ -85,13 +85,58 @@ const post = await reddit.submitCustomPost({
 
 ## Update a post’s text fallback
 
-The post author can edit and update text fallback content after it’s been created. To do this, call post.setTextFallback with the desired fallback content.
+Call `post.setTextFallback()` to update the fallback content of an existing custom post. The method uses the app identity by default. For an app-authored post, omit `runAs` or set it to `"APP"`:
 
 ```tsx
-import { reddit } from '@devvit/web/server';
+import { context, reddit } from "@devvit/web/server";
+
+export async function updateAppFallback() {
+  if (!context.postId) {
+    throw new Error("Post context required.");
+  }
+
+  const post = await reddit.getPostById(context.postId);
+  await post.setTextFallback({ text: "Updated instructions for this post." });
+}
+```
+
+### Update a user-authored post
+
+Starting with Devvit 0.14.3, you can pass `runAs: "USER"` to update fallback content on behalf of the current user. Enable the `SUBMIT_POST` scope:
+
+```json title="devvit.json"
+{
+  "permissions": {
+    "reddit": {
+      "enable": true,
+      "asUser": ["SUBMIT_POST"]
+    }
+  }
+}
+```
+
+Call this function from a signed-in post interaction. This example checks that the current user is the post author before requesting an update:
+
+```tsx
+import { context, reddit } from "@devvit/web/server";
+
+export async function updateUserFallback() {
+  if (!context.userId || !context.postId) {
+    throw new Error("Signed-in post interaction required.");
+  }
+
+  const post = await reddit.getPostById(context.postId);
+  if (post.authorId !== context.userId) {
+    throw new Error("Post author required.");
+  }
 
-// from a menu action, form, scheduler, trigger, custom post click event, etc
-const newTextFallback = { text: 'This is an updated text fallback' };
-const post = await reddit.getPostById(context.postId);
-await post.setTextFallback(newTextFallback);
+  await post.setTextFallback({
+    text: "Updated instructions for this interactive post.",
+    runAs: "USER",
+  });
+}
 ```
+
+`"USER"` uses the current request's user. It cannot select an arbitrary user or bypass Reddit's edit permissions. User grants and authorization still apply, and a scheduler or trigger handler may not have a suitable acting-user context.
+
+The method returns `Promise<void>` and rejects if the update fails. Await it before reporting success. See [SetCustomPostTextFallbackOptions](../../api/redditapi/models/type-aliases/SetCustomPostTextFallbackOptions.md) for the content options and `runAs` type.