feat(YALB-1233): Update post tag help text

Update post tag help text to convey that they cannot create tags while
creating a post.

web/profiles/custom/yalesites_profile/config/sync/field.field.node.post.field_tags.yml

@@ -11,7 +11,7 @@ field_name: field_tags
 entity_type: node
 bundle: post
 label: Tags
-description: 'Separate tags with commas. Tags will autocomplete if one exists.'
+description: 'Tags will autocomplete as you type.  Choose from pre-created options only.  To add a new tag, go to ''Content->Manage Taxonomy->Tags''.'
 required: false
 translatable: true
 default_value: {  }