doc.xotcl

- I created a first draft of the nx language manual, based on the new next::doc facilities. It is still incomplete, but demonstrates the use of next::doc for authoring code documentation.
To re-create the language reference (which is not yet integrated into
the build environment), run:

./nextsh tests/doc.xotcl

You will then find an output directory "NextLanguageCore" in your
/tmp/ directory.
- The next::doc comments which are sourced for generating the manual
can be found in generic/gentclAPI.decls and
generic/predefined.xotcl. I tried to add most comments to the
former, as the complexity of the predefined script does not comfort
documentation comments (and vice versa).
- Applied many fixes to the templates (based on the needs of the
language reference)