Add documentation comments to standard library of Tact, `stdlib/`

[novusnota]

They will help various tools, especially editor plugins and language servers, to show additional information in hover requests and make go to definitions look just a little bit better. This change will significantly reduce the maintenance burden on their side, while allowing core and community contributors to the Tact compiler to keep them up to date with every PR that changes or adds something to Tact's standard library, stdlib/.

As a bonus, such documentation comments might even serve as an offline reference to some extent, for anyone wandering around in node_modules/ :)