Element should have a "design discussion" section talking about build order

[Hixie]

We should document some of our design decisions about how widgets build:

• The framework guarantees that ancestors build before descendants.
• The framework does not guarantee anything about the order other than this.
• Some widgets might in rare cases build twice, this is considered an aberration to be avoided (layout builder that ends up dirty both at the widget layer and the layout layer is the example).
• One pass build.

Maybe point to "Inside Flutter" (https://flutter.dev/docs/resources/inside-flutter).

[Hixie]

cc @rrousselGit @goderbauer

[pedromassangocode]

Can't find any reference about the topics mentioned above in the Inside Flutter link, so I assume this still valid.