docs(toml): complete documentation

[sylc]

towards #3764

[sylc]

The toml/mod module comment has an @example. But i'm not sure how useful that is from within intellisense. Should the example be duplicated on the 2 functions parse and stringify themselves ?
(also the main module comment uses markdown. is that ok?)

[kt3k]

Should the example be duplicated on the 2 functions parse and stringify themselves ?

Duplicated example for parse and stringify functions are also useful in my view. Such examples will appear in html doc pages like this one https://deno.land/std@0.212.0/toml/mod.ts?s=parse

also the main module comment uses markdown. is that ok?

Markdown comments are fine. They are rendered nicely in html doc pages like this page.

[kt3k]

LGTM. Thanks!