doc: update doc-comments for latest LDoc master.

* doc/config.ld (metamethod): Delete. Metamethods are now
separated out automatically.
* ext/std/list.lua: Describe use of object methods as module
functions.
Update doc-comments.
* ext/std/object.lua, ext/std/set.lua, ext/std/strbuf.lua,
ext/std/strict.lua, ext/std/tree.lua: Update doc-comments.

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