build(docs-infra): improve directive selector rendering (#25768)

If the documentation contains a `@selectors` tag then the content of that
is used to describe the selectors of a directive.

Otherwise the selector string is split and each selector is listed as
a list item in an unordered list.

PR Close #25768

aio/tools/transforms/angular-api-package/tag-defs/selectors.js

@@ -0,0 +1,3 @@
+module.exports = function() {
+  return {name: 'selectors'};
+};