docs: add Fern Writer capabilities section

fern/products/docs/pages/ai/writer.mdx

@@ -26,6 +26,41 @@ Request changes by commenting in the Slack thread or directly on the PR. Fern Wr
 **Coming soon**: Automatic documentation updates based on code changes and releases.
 </Note>
 
+## Capabilities
+
+Fern Writer goes beyond simple text edits. It understands your documentation structure, components, and style, letting it handle a wide range of documentation tasks.
+
+### Write and update content
+
+Fern Writer can create new pages, add sections to existing pages, and update content across your docs. It works with Markdown and MDX, including Fern's full [component library](/learn/docs/writing-content/components/overview) — Callouts, Steps, Tabs, Cards, Code blocks, Accordions, and more. It generates syntactically correct components and follows your existing content patterns.
+
+When you reference a GitHub pull request or issue, Fern Writer reads it to understand the changes and translates them into documentation updates.
+
+### Reorganize and restructure
+
+Fern Writer can move, merge, and split pages across your docs. It updates your navigation configuration in `docs.yml` and adds [redirects](/learn/docs/seo/redirects) so existing links continue to work. This is useful when consolidating related pages, splitting a long page into focused guides, or restructuring a section.
+
+### Review and fix content
+
+Fern Writer can audit your docs for issues such as broken code examples, outdated versions, inconsistent terminology, and unclear instructions. You can ask it to review a specific page or an entire section, and it will create a PR with targeted fixes.
+
+### Match your writing style
+
+Fern Writer analyzes your existing documentation to match your tone, terminology, and conventions. To give it more specific guidance, add an `AGENTS.md` file to your docs repository with style rules, preferred terminology, formatting standards, and any other instructions. Fern Writer reads this file before every task.
+
+### Iterate on feedback
+
+You can request changes in two ways:
+
+- **Slack thread**: Reply in the same thread where you made the original request. Fern Writer reads the full conversation for context.
+- **PR comments**: Leave review comments directly on the pull request. Fern Writer responds to feedback from both humans and automated review tools like linters or CI checks.
+
+Fern Writer pushes additional commits to the same PR, so you can track the full history of changes.
+
+### Attachments and context
+
+Fern Writer supports image and file attachments in Slack for additional context. Share screenshots, design specs, or reference documents alongside your request. When tagged in an existing thread, it reads the full conversation history before responding.
+
 <Accordion title="Video example">
 <video
     src="fern-writer.mp4"