docs(file-uploader): improve FileUploader documentation (#1126)

- add `FileUploader` "Clear files" example
- document `validateFiles` prop for `FileUploaderDropContainer`
- add descriptions for `FileUploaderButton`, `FileUploader` examples

docs/src/pages/framed/FileUploader/FileUploaderClearFiles.svelte

@@ -0,0 +1,32 @@
+<script>
+  import { FileUploader, Button } from "carbon-components-svelte";
+
+  let fileUploader;
+  let files = [];
+</script>
+
+<FileUploader
+  bind:this="{fileUploader}"
+  multiple
+  labelTitle="Upload files"
+  buttonLabel="Add files"
+  status="complete"
+  bind:files
+/>
+
+<br />
+
+<Button
+  kind="tertiary"
+  disabled="{!files.length}"
+  on:click="{fileUploader.clearFiles}"
+>
+  Clear (programmatic)
+</Button>
+<Button
+  kind="tertiary"
+  disabled="{!files.length}"
+  on:click="{() => (files = [])}"
+>
+  Clear (two-way binding)
+</Button>