This introduces a new `Doc` object that can be owned by `Expr` (only atm)
and contain what it describes as a concise independent clause. This
construction is not enforced, and is only really obvious today via the
Summary Pages.
There's a lot of latent and unrealized potential in TAME's documentation
philosophy that was never realized, so this will certainly evolve over
time. But for now, the primary purpose was to get `@desc` working on things
like classifications so that `xmli` output can compile for certain
packages.
DEV-13708
Mike Gerwitz