docs: Add some FAQ entries

docs/guides/faq.md

@@ -41,6 +41,9 @@
 * [Q: Can Video.js be required in node.js?](#q-can-videojs-be-required-in-nodejs)
 * [Q: Does Video.js work with webpack?](#q-does-videojs-work-with-webpack)
 * [Q: Does Video.js work with react?](#q-does-videojs-work-with-react)
+* [Q: Can the big play button be centered?](#q-can-the-big-play-button-be-centered)
+* [Q: Can the big play button be shown when paused?](#q-can-the-big-play-button-be-shown-when-paused)
+* [Q: Why is the picture-in-picture button different in Firefox?](#q-why-is-the-picture-in-picture-button-different-in-firefox)
 
 ## Q: What is Video.js?
 
@@ -273,6 +276,18 @@ Yes! See the [Webpack and Video.js configuration guide][webpack-guide].
 
 Yes! See [ReactJS integration example][react-guide].
 
+## Q: Can the big play button be centered?
+
+The default skin offsets the button to not obscure the poster image, but just add a `vjs-big-play-centered` class to the player to have it centred.
+
+## Q: Can the big play button be shown when paused?
+
+Add a `vjs-show-big-play-button-on-pause` class to the player to display the button when paused.
+
+## Q: Why is the picture-in-picture button different in Firefox?
+
+Firefox does not support the HTML video Picture-in-Picture (PIP) API, so it is not possible to have a custom button in the Video.js controls. Firefox has its own overlay PIP button, and its [own logic for whether to display it][firefox-pip].
+
 [ads]: https://github.com/videojs/videojs-contrib-ads
 
 [audio-tracks]: /docs/guides/audio-tracks.md
@@ -293,6 +308,8 @@ Yes! See [ReactJS integration example][react-guide].
 
 [eme]: https://github.com/videojs/videojs-contrib-eme
 
+[firefox-pip]: https://firefox-source-docs.mozilla.org/toolkit/components/pictureinpicture/pictureinpicture/index.html#the-picture-in-picture-toggle
+
 [flash-eol]: https://www.adobe.com/products/flashplayer/end-of-life.html
 
 [generator]: https://github.com/videojs/generator-videojs-plugin

docs/guides/troubleshooting.md

@@ -14,6 +14,7 @@
 * [Problems with playback](#problems-with-playback)
 * [Video.js Errors](#videojs-errors)
   * [vdata123456 errors](#vdata123456-errors)
+* [Problems with setup options](#problems-with-setup-options)
 
 ## Problems with media formats
 
@@ -89,6 +90,24 @@ a component.
 
 To fix this issue please make sure that all event listeners are cleaned up on dispose.
 
+## Problems with setup options
+
+If a player is inititalized without the expected setup options, it's usually because of using both the `data-setup` attribute and the `videojs()` constructor. Take this example:
+
+```html
+<video-js id="my_video" data-setup='{"autoplay": "any"}'></video-js>
+
+<script>
+  const myPlayer = videojs('my_video');
+</script>
+```
+
+Here you might expect the player to be initialized with the `autoplay` option set, but it will not be. While Video.js sets up player embeds that have a `data-setup` attribute automatically when the page is loaded, in this case the player will have _already been setup_ in the script tag. The `data-setup` option will never be applied.
+
+This can be more confusing if `videojs('my_video')` is used in an async script as different behavior will occur depending on when the script is executed.
+
+It's better to not use `data-setup` and use the `videojs()` constructor once to set up the player, and/or only use the explicit getter `videojs.getPlayer('my_video')` to get a player reference.
+
 [hosting-media]: #problems-when-hosting-media
 
 [text-tracks]: /docs/guides/text-tracks.md