Add the optionable class to ScopeInfo. #docfixit

Review Request #2588 — Created Aug. 10, 2015 and submitted — Latest diff uploaded

stuhood, zundel

Not all scopes have corresponding optionables (e.g.,
intermediate scopes don't), but most do, and in those cases
it would be convenient to wrap the optionable class up in
the ScopeInfo. E.g., help generation can get scope-wide
description text from it.

While doing this, simplified the set of categories (we don't
actually need the distinction between TASK_SUBSYSTEM and
GLOBAL_SUBSYSTEM), which also allowed the optionable class
to create its own ScopeInfo, instead of invoking the ScopeInfo
ctor directly all over the place, with redundant arguments.

[Note: Diff is on top of]

CI passes: