Added documentation for proof terms.
Isabelle Reference Manual. Report 283
This manual is a comprehensive description of Isabelle, including all
commands, functions and packages. Functions are organized according to the
task they perform. In each section, basic functions appear before advanced
ones. The Index provides an alphabetical listing. It is intended as a
reference, not for casual reading. The manual assumes familiarity with the
basic concepts explained in Report 280, Introduction to Isabelle.