📚 Documentation: Update InputFile documentation about what to use for Flutter Web

### 💭 Description

On Flutter Web, you must use `InputFile.fromBytes()`. This was fine in the past because Flutter file/image picker packages didn't provide a path. They've added support for path now, but the path is an [Object url](https://developer.mozilla.org/en-US/docs/Web/API/URL/createObjectURL_static) and not an actual image path. As such, this path can't be used by the Appwrite SDK.

### 👀 Have you spent some time to check if this issue has been raised before?

- [X] I checked and didn't find similar issue

### 🏢 Have you read the Code of Conduct?

- [X] I have read the [Code of Conduct](https://github.com/appwrite/appwrite/blob/HEAD/CODE_OF_CONDUCT.md)