Update docs for sharp usage to mention Vercel (#28476)

There was some confusion in [this tweet](https://twitter.com/mtliendo/status/1430269174459338759) so I revised the docs around `sharp` usage.

docs/basic-features/image-optimization.md

@@ -128,7 +128,7 @@ If you need a different provider, you can use the [`loader`](/docs/api-reference
 
 > The `next/image` component's default loader is not supported when using [`next export`](/docs/advanced-features/static-html-export.md). However, other loader options will work.
 
-> The `next/image` component's default loader uses the ['squoosh'](https://www.npmjs.com/package/@squoosh/lib) library for image resizing and optimization. This library is quick to install and suitable for a dev server environment. For a production environment, it is strongly recommended that you install the optional [`sharp`](https://www.npmjs.com/package/sharp) library by running `yarn add sharp` in your project directory. If sharp is already installed but can't be resolved you can manually pass the path to it via the `NEXT_SHARP_PATH` environment variable e.g. `NEXT_SHARP_PATH=/tmp/node_modules/sharp`
+> The `next/image` component's default loader uses [`squoosh`](https://www.npmjs.com/package/@squoosh/lib) because it is quick to install and suitable for a development environment. When using `next start` in your production environment, it is strongly recommended that you install [`sharp`](https://www.npmjs.com/package/sharp) by running `yarn add sharp` in your project directory. This is not necessary for Vercel deployments, as `sharp` is installed automatically.
 
 ## Caching
 

errors/sharp-missing-in-production.md

@@ -2,11 +2,16 @@
 
 #### Why This Error Occurred
 
-The `next/image` component's default loader uses the ['squoosh'](https://www.npmjs.com/package/@squoosh/lib) library for image resizing and optimization. This library is quick to install and suitable for a dev server environment. For a production environment, it is strongly recommended that you install the optional [`sharp`](https://www.npmjs.com/package/sharp). This package was not detected when leveraging the Image Optimization in production mode (`next start`).
+The `next/image` component's default loader uses [`squoosh`](https://www.npmjs.com/package/@squoosh/lib) because it is quick to install and suitable for a development environment. For a production environment using `next start`, it is strongly recommended you install [`sharp`](https://www.npmjs.com/package/sharp) by running `yarn add sharp` in your project directory.
+
+You are seeing this error because Image Optimization in production mode (`next start`) was detected.
 
 #### Possible Ways to Fix It
 
-Install `sharp` by running `yarn add sharp` in your project directory.
+- Install `sharp` by running `yarn add sharp` in your project directory and then reboot the server by running `next start` again
+- If `sharp` is already installed but can't be resolved, set the `NEXT_SHARP_PATH` environment variable such as `NEXT_SHARP_PATH=/tmp/node_modules/sharp next start`
+
+> Note: This is not necessary for Vercel deployments, since `sharp` is installed automatically for you.
 
 ### Useful Links