table: complete specs and improve LDocs.

* lib/std/table.lua (clone, clone_select, empty, invert, keys)
(merge, merge_select, monkey_patch, new, pack, ripairs, size)
(sort, totable, values): Add argcheck calls, and improve LDocs
with usage examples & cross-references.
(clone, clone_select, merge, merge_select): Minimise overhead
when argchecking is disabled by wrapping complex argument
checking in `if init._ARGCHECK`.
* specs/table_spec.yaml (pack, ripairs, totable): Add missing
specifications.
(clone, clone_select, empty, invert, keys, merge, merge_select)
(monkey_patch, new, size, sort, values): Improve specs to match
argcheck behaviours.

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