Add footnote/reference to list of enum values

[trask]

From @jonatan-ivanov's https://github.com/open-telemetry/semantic-conventions/pull/297/files#r1364675468

I'm thinking if it would worth adding some kind of and indicator (footnote) for the thread state examples that shows where is the rest of that list in case there will be more, e.g.:

runnable; blocked [1]

[1]: jvm.thread.state MUST be one of the following:
...