Document the design decisions and up-to-date behaviors of the importer and exporter.

Here is some discussion copied from an email chain:


foolip@:
One thing that I realized is that we don't have an up-to-date design doc or explainer for how our import/export process actually works. We started out with https://docs.google.com/document/d/1JgPTyIWmjlXyhyatiZ9A6fSbUQPjCyM26budEY90VDE/edit?usp=sharing but it doesn't match how things have evolved.

What do you think about updating this, or writing a new doc, some time in the coming months? It could include justification for some of the choices we've made, like why we export out of order and reapply patches when importing. I think that could be useful to WebKit, Gecko and Edge.


robertma@:
In addition to how they work, we can probably put "expected" failure modes and the manual process needed in each case into the doc (i.e. part of the rotation doc). I would prefer putting the doc in chromium/src/docs. I personally find it easier to discover.


qyearsley@:
The most up-to-date documentation is currently //docs/testing/web_platform_tests.md. The target audience for that doc is Chromium developers in general, so it doesn't go into a lot of detail about specifics about the import and export process.


robertma@:
I think it's a good idea to separate general docs for "users" (other chromium developers) and "developers" (people working on wpt infra), at least putting them in two chapters.


foolip@:
I guess that https://chromium.googlesource.com/chromium/src/+/master/docs/design/ would be the natural place for that.