clean up some writing in the spec (ch1, ch2, ch11)

[gavinking]

This pull request contains a raft of changes to improve the readability and understandability of the spec.

It focuses on important parts of ch2 and ch11.

A future pull request will do the same work for chapter 3, and, possibly, for ch7.

Note that:

• ch4 is going away;
• chs 6 and 12 are uninteresting;
• chs 8 and 9 seem fine more or less as they are, even though ch9 has a weird structure.

So I don't plan to invest time in rewriting these chapters.

But chapters 2 and 3 in particular are very important and deserve a long-overdue refresh.