Improve documentation in restructuring/behavior

Use coqdoc's header feature to make it easier to find key concepts and
improve/tweak a few comments.
Status Job ID Name Coverage
  Build
passed #47793
1.9.0-coq-8.10

00:02:52

passed #47792
1.9.0-coq-8.9

00:02:48

passed #47794
allowed to fail
1.9.0-coq-dev

00:02:38

passed #47795
allowed to fail
latest-coq-8.10

00:01:45

 
  Process
passed #47797
doc

00:03:35

passed #47798
proof-length

00:00:14

passed #47796
validate

00:01:23