Add inline documentation to evalR

[JosiahParry]

I tried using evalRString() in the hopes that it would return the results as a string. However, it expects a string and will error if a string is not returned from the function.

It would be really nice if this expectation was documented in the API reference
https://docs.r-wasm.org/webr/latest/api/js/classes/WebR.WebR.html#evalrstring

webr/src/webR/webr-main.ts

Lines 416 to 418 in c07d899

	async evalRString(code: string, options?: EvalROptions) {
	return this.evalRRaw(code, 'string', options);
	}