User's Manual text for reveal.js background images

[Jmuccigr]

Following up on #3979…

How does this look for instructions:

---

Background images can be added to self-contained reveal.js slideshows.

• For the same image on every slide, use the reveal.js configuration option parallaxBackgroundImage either in the YAML metadata block or as a command-line variable. You can also set the parallaxBackgroundHorizontal and parallaxBackgroundVertical variables the same way and must also set parallaxBackgroundSize to have your values take effect. If you include a size, but do not set the horizontal or vertical values, reveal.js will apply its own.

• To set the image for a particular slide, add the following attribute to the first slide-level header on the slide (which may even be empty): {data-background-image="/path/to/image"}. This will override any parallaxBackgroundImage for this slide.

Note that in reveal.js's overview mode, the background image will show up only on the first slide, while a data-background-image will show up each time it is used.

For example:

---
title: My Slideshow
parallaxBackgroundImage: /path/to/my/background_image.png
---

## Slide One

Here's my first slide. It has background_image.png as its background.

## Slide Two {data-background-image="/path/to/special_image.jpg"}

My second slide has a special image for its background.

## Slide Three

Back to my first background image!

## {data-background-image="/path/to/special_image.jpg"}

Slide Four has a special image for its background, even though the header has no content.

[Jmuccigr]

I've now created a PR for this with a few more details added. #4325

[Jmuccigr]

@jgm, thoughts...or just low on the priority list? :-)