Update gssp-export Error (vercel#15529)

This pull request updates the `gssp-export` error for clarity.

Fixes vercel#15527

Author: Timer

errors/gssp-export.md

@@ -2,13 +2,37 @@
 
 #### Why This Error Occurred
 
-You attempted to export a page with `getServerSideProps` which is not allowed. `getServerSideProps` is meant for requesting up to date information on every request which means exporting it defeats the purpose of the method.
+You attempted to statically export your application via `next export`, however, one or more of your pages uses `getServerSideProps`.
+
+The `getServerSideProps` lifecycle is not compatible with `next export`, so you'll need to use `next start` or a [serverless deployment](https://vercel.com).
 
 #### Possible Ways to Fix It
 
-If you would like the page be static you can leverage the `getStaticProps` method instead.
+1. If you'd like to keep your application static, you can use `getStaticProps` instead of `getServerSideProps`.
+
+2. If you want to use server-side rendering, update your build command and remove `next export`. For example, in your `package.json`:
+
+   ```diff
+   diff --git a/bla.json b/bla.json
+   index b84aa66c4..149e67565 100644
+   --- a/bla.json
+   +++ b/bla.json
+   @@ -1,7 +1,7 @@
+   {
+     "scripts": {
+       "dev": "next dev",
+   -    "build": "next build && next export",
+   +    "build": "next build",
+       "start": "next start"
+     }
+   }
+   ```
+
+> **Note**: Removing `next export` does not mean your entire application is no longer static.
+> Pages that use `getStaticProps` or [no lifecycle](https://nextjs.org/docs/advanced-features/automatic-static-optimization) **will still be static**!
 
 ### Useful Links
 
+- [Automatic Static Optimization](https://nextjs.org/docs/advanced-features/automatic-static-optimization)
 - [`getStaticProps` documentation](https://nextjs.org/docs/basic-features/data-fetching#getstaticprops-static-generation)
 - [`exportPathMap` documentation](https://nextjs.org/docs/api-reference/next.config.js/exportPathMap)