debug: complete specs and improve LDocs.

* lib/std/debug.lua (__call, _DEBUG, argcheck, argerror)
(argscheck, say, trace): Improve LDocs with usage examples and
cross-references.
* specs/debug_spec.yaml (_DEBUG): Remove. _DEBUG behaviours are
specified in the api calls that are affected by it.
(say, trace): Add missing specificatons.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>