Tweak documentation for readability

[PoignardAzur]

Apply many small tweaks I found while going over the documentation.
Rename some variables.
Add asserts to BoxConstraints::constrain_aspect_ratio.
Flesh out the WidgetState documentation.
Introduce the concept of "zombie flags".

[PoignardAzur]

I'd say this is ready for review.

It's a bit of a kitchen sink PR, but almost all the changes are to documentation or comments only, and almost none of them change executable code.

Some changes remove long-dead code, bitrotted comments, outdated documentation, etc. Masonry's documentation has a tendency to refer to old ideas as if we were just about to implement them, so I think this spring cleaning will make it more readable.

[jaredoconnell]

This is an improvement.