Doc: use conventional comment style in JavaScript examples

[smorimoto]

The manual consistently wrote runtime annotation comments without a space after // (e.g. //Provides:), and a couple of JavaScript examples used var. This updates the documentation to a more conventional style:

• Use the spaced form // Provides: (and // Requires:, // Version:, etc.) for runtime annotations. Both forms are accepted by the linker; the spaced form is more idiomatic for JavaScript comments.
• Use const instead of var in the JavaScript examples.

Only the documentation is touched (manual/*.mld, the Js.Unsafe.runtime_value doc comment, and a CHANGES.md entry). The runtime sources themselves are left unchanged.

[smorimoto]

Related: ocamllabs/vscode-ocaml-platform#2162