Relax the typing on engine and flag names

There is no real reason these names/keys cannot be a general string.
Add references to section 4 to some sections

These references were added to sections that contain a section 4
reference in the syntax definition. These links should make it
easier to navigate.
Change section 3.2.1

Made the section title text and production name consistent with
the message identifier.
Improved the description of how message 3.2.1 is used
Improve section 4

Show more clearly how the specific definitions in this section
relate back to the general definitions in the other sections.