Make the linenumbers config.toml changes opt-in

Unfortunately the config.toml is shared by all the examples and so we
try to push example-specific config into the index files. With markdown
configuation that's not possible, so the best we can do is tell the user
they need to comment it in to try it out. The better solution in the
future would be to pull this example out into it's own Hugo site.

exampleSite/content/highlightjs-linenumbers-example/_index.md

@@ -7,17 +7,17 @@ theme = "white"
 highlight_theme = "vs"
 +++
 
-## New highlighting features Presentation
+## Multiline highlighting with Highlight.js
 
 ---
 
-This presentation shows the use of the [new highlighting features](https://github.com/hakimel/reveal.js/blob/master/README.md#step-by-step-highlights) which were introduced with Reveal.js [v3.9.0](https://github.com/hakimel/reveal.js/releases/tag/3.9.0)
+This presentation shows the use of the [new highlighting features](https://github.com/hakimel/reveal.js/blob/master/README.md#step-by-step-highlights) which were introduced with Reveal.js [v3.9.0](https://github.com/hakimel/reveal.js/releases/tag/3.9.0).
 
 ---
 
 ## Prerequisite
 
-First, disable `CodeFences` in to your `config.toml`:
+Disable `codeFences` in to your `config.toml` to prevent Hugo's built-in highlighting.
 
 {{< highlight toml "style=github" >}}
 [markup.highlight]
@@ -25,6 +25,11 @@ codeFences = false
 {{< /highlight >}}
 
 ---
+
+❗ To properly view the rest of this example presentation with Highlight.js highlighting, you must now uncomment the line to disable `codeFences` in the `exampleSite/config.toml` file.
+
+---
+
 ## Theme
 
 Specify a theme for Highlight.js in `config.toml`
@@ -34,7 +39,9 @@ Specify a theme for Highlight.js in `config.toml`
 highlight_theme = "github"
 {{< /highlight >}}
 
-or in the `frontmatter`
+---
+
+...or in the `_index.md` front matter
 
 {{< highlight toml "style=github" >}}
 [reveal_hugo]
@@ -138,8 +145,20 @@ However it can be achieved by adding the some [custom CSS](https://github.com/dz
 </style>
 {{< /highlight >}}
 
-<small>💡 Tip: To hide line numbers for every presentation, put it here:</small>
+---
+
+💡 Put the custom CSS in either of these partials:
 
 ```text
+# for all presentations
 layouts/partials/reveal-hugo/body.html
-```
+```
+
+```text
+# for one presentation
+layouts/partials/{presentation-name}/reveal-hugo/body.html
+```
+
+---
+
+Thanks!