PKG: Edits to terms and definitions

I've just completed a pass through the UBL 1.0 terms and
definitions (section 3) and made the following changes:

Removed because not referenced anywhere in the normative parts of
the document:

- business context

- container

- containership

Removed because its definition does not accurately represent the
way the term is used in the document:

 - document assembly

Removed because the distinction can be made more clearly in the
text:

 - schema (lowercase "s")

Added:

 - XSD schema

I've also made some edits to Section 4 (symbols and
abbreviations), but nothing worth logging.

Jon