add help text to issue entityreference fields on issues to explain how the autocomplete can be used

The autocomplete on the entityreference fields for parent and related issues work with a variety of things for the autocomplete, but I bet a lot of people don't realize this and just search on partial title.

We should add field help text.

How about:

> Search for issues by entering either a URL, a node ID prefixed with '#', or a partial title.