Add some JSDoc comments to rustdoc JS

[jsha]

This follows the Closure Compiler dialect of JSDoc, so we can use it to do some basic type checking. We don't plan to compile with Closure Compiler, just use it to check types. See https://github.com/google/closure-compiler/wiki/ for details.

To try checking the annotations, run:

npm i -g google-closure-compiler
google-closure-compiler -W VERBOSE build/x86_64-unknown-linux-gnu/doc/{search-index1.59.0.js,crates1.59.0.js} src/librustdoc/html/static/js/{search.js,main.js,storage.js} --externs src/librustdoc/html/static/js/externs.js >/dev/null

You'll see some warnings that "String continuations are not recommended". I'm not addressing those right now.

Discussed on Zulip.

r? @GuillaumeGomez