Provide additional details on how SpeedIndex is calculated

**Feature request summary**

Currently, the information LH provides around SpeedIndex is not very actionable.
Some reasons might include:

- Lack of better visualization in filmstrip view (see #5844).
- There are no hints at exactly which point the SpeedIndex is calculated. Perhaps a marking the before / after screenshot at the exact point., or coloring the screenshot that triggers the metric might help.
- It's not clear if things like permission prompts can affect Speed Index.

Example: Testing https://www.autotrader.com.

**Test 1: Remote LH on Web.dev ([report](https://lighthouse-dot-webdotdevsite.appspot.com/lh/html?url=https://www.autotrader.com#performance)):**

At the moment of writing this the report showed a value of 13s.

In the Filmstrip view, the site goes from a blank page, to almost visually complete, which suggests that, SpeedIndex should be close to FCP (1.6s at the time of this writing), but, as seen, it's not the case.

As a result, the developer doesn't know exactly which part of the page to optimize to improve this metric.

**Test 2: LH over WPT:**

This [LH Report](https://www.webpagetest.org/lighthouse.php?test=190614_EZ_da0a4064e41f51096d83818de14540e9&run=5) was ran at the same time than the Web.dev one, and shows a value of SpeedIndex of 3s (3X less).

In this case, it's not clear why SpeedIndex has such value (and why it's so different from the one ran simultaneously in Web.dev/measure).

**Test 3: Traditional WPT:**

The [WPT report](https://www.webpagetest.org/result/190614_EZ_da0a4064e41f51096d83818de14540e9/) associated with the previous LH report, shows a SpeedIndex of 14.9s (close to the one on the initial LH report from Web.dev).

The [FIlmstrip view](https://www.webpagetest.org/video/compare.php?tests=190614_EZ_da0a4064e41f51096d83818de14540e9-r:5-c:0), shows that the visual completeness goes from 57% to 100%, due to the permission prompt, which might explain this value.

**What is the motivation or use case for changing this?**

Developers usually look for ways to improve individual metrics, in order to increase their overall performance score, and lead to better user experiences.
The only information about SpeedIndex is in [this article](https://developers.google.com/web/tools/lighthouse/audits/speed-index?utm_source=lighthouse&utm_medium=unknown), but it's hard to know how to optimize it.

**How is this beneficial to Lighthouse?**

Adding more clarity, would make the Lighthouse report more actionable, on one of the main metrics taken into account to generate the overall performance score.
Also, the community often uses Filmstrip views as a tool to diagnose performance problems (this [tweet](https://twitter.com/tameverts/status/1138523573813833728) is a good example of that).