Nunjucks and async filters - clearer documentation

[cfjedimaster]

Unless I'm seeing wrong, while Nunjucks does support async filters, it does not support them if you do it via a global filter. I had this:

eleventyConfig.addFilter("qrcode", async function(value) {
		return await qrCode.toDataURL(value);
});

Which worked fine in Liquid, but returned a pending promise in Nunjucks. Adding this made it work in Nunjucks:

eleventyConfig.addNunjucksAsyncFilter("qrcode2", async function(value, callback) {
	let result = await qrCode.toDataURL(value);
	callback(null,result);
});

I guess my issue here is - it doesn't seem clear to me that the universal async filter should fail in Nunjucks. Can we add something to the doc (I'd say to both pages) to clarify this? (Willing to file a PR.)

[zachleat]

https://www.11ty.dev/docs/filters/#asynchronous-universal-filters