This adds some information on the two broad categories of sections:

Container Sections (those with subsections but no content)
Content Sections (those with content but no subsections)

The categories link to every section of the given type, and each section

has information identifying its type.
Subsection lists also point out which subsections are optional and which

are required.

Built the docs. Checked the new text, made sure all links worked, and

that each section had an identifying type.