docs(typescript): add host configuration to rootDirs

[dgp1130]

A host root directory is necessary for TypeScript to resolve imports for a tool executed at build time with cfg = "host".

This is necessary for:

some_rule = rule(
    implementation = _some_rule_impl,
    attrs = {
        "_binary": attr.label({
            default: "//some/nodejs_binary/tool/built/with/ts_project",
            executable: True,
            cfg = "host",
        }),
    },
)

Because Bazel will build the tool in the host configuration, meaning generated *.js files will be dropped into bazel-out/host/bin/..., and tsc needs to know to look for dependent ts_project() outputs in that directory.

Arguably a similar change is also necessary for cfg = "exec", however files would be in bazel-out/k8-opt-exec-*/bin, which can't be easily hard-coded in a tsconfig, so that's a problem to be tackled separately.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Documentation content changes

Does this PR introduce a breaking change?

• Yes
• No

Other information

While not a breaking change, many users are likely to have copy-pasted this content from the docs and will not easily discover this information or update their tsconfig.json file. This could add additional confusion due to a difference between new projects which copy-paste this content, and old projects which already did.

[mattem]

Thanks, I think it's okay to point users to updated docs as an issue is discovered