doc: improve object LDocs, and add usage examples.

* lib/std/object.lua: Add usage examples to apis.
(clone): Normally we'd use the __call metamethod to clone from a
given object, so mark the LDocs for clone as @static because it
is primarily a module function.
(prototype): Mark the function documentation as @static, and add
equivalent method documentation.

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