Document modules and structures in the build system

Give a brief description of each of the modules that comprises the build
system, and the suffices that is used to form a virtual structure of
variables for each project.
Terminology instead of Notation as section header

Notation implies that we are going to be given symbols which
represents values or such (which arguably could be true since the
path is a particular value, and from programming we are used to
multi-letter symbols perhaps also with whitespace, but it's a
stretch), but Terminology is more accurate, as this section
describes fits the definition better: it is a vocabulary of
technical terms.

Hat tip: @bska
Merge pull request #456 from rolk/456_blddoc

Document modules and structures in the build system