feat(popper): arrow option & docs consistency across components (#875)

docs/components/content/examples/DropdownExamplePlacement.vue

@@ -0,0 +1,16 @@
+<script setup>
+const items = [
+  [{
+    label: 'Profile',
+    avatar: {
+      src: 'https://avatars.githubusercontent.com/u/739984?v=4'
+    }
+  }]
+]
+</script>
+
+<template>
+  <UDropdown :items="items" :popper="{ placement: 'right-start' }">
+    <UButton color="white" label="Options" trailing-icon="i-heroicons-chevron-down-20-solid" />
+  </UDropdown>
+</template>