Update directive documentation with tip relating to v-html and scoped styles (#1259)

* Update index.md

Adding a tip relating to the fact that v-html content is unaffected by scoped styling, and explaining an example way around it.

* Revise note about v-html and scoped styles

Author: Dave Goodchild

src/v2/api/index.md

@@ -1686,6 +1686,8 @@ type: api
 
   <p class="tip">Dynamically rendering arbitrary HTML on your website can be very dangerous because it can easily lead to [XSS attacks](https://en.wikipedia.org/wiki/Cross-site_scripting). Only use `v-html` on trusted content and **never** on user-provided content.</p>
 
+  <p class="tip">In [single-file components](../guide/single-file-components.html), `scoped` styles will not apply to content inside `v-html`, because that HTML is not processed by Vue's template compiler. In you want to target `v-html` content with scoped CSS, you can instead use [CSS modules](https://vue-loader.vuejs.org/en/features/css-modules.html) or an additional, global `<style>` element with a manual scoping strategy such as BEM.</p>
+
 - **Example:**
 
   ```html