We have documentation on the contents of the velocity context for email notifications and custom fields, we need one for release notes. From a user:

What i had trouble with tho was not the velocity side as it is relatively straight forward, but more so the fact that i had no idea what classes i had access to from within the velocity template, or how to add classes (infact i didn't even know you could add classes).

Once i got over that hurdle, it was just a matter of finding the relevant classes to pass to the release notes in the jira api, so i could gain acces to each issues linked issues.

I think that some documentation could be added to this affect. ie what classes are initially available to the release notes templates. As well as how to add more classes to this list (through the method getTemplateVariables in ReleaseNoteManager)
Another issue, which fortunately i didn't come up against as we have the enterprise edition of jira so was able to get the source code for the ReleasenotesManager, edit it, recompile and replace the existing ReleaseNoteManager class file. But if i didn't have enterprise and didn't have access to the source, how would have i gone about this?

Anyway that just my 2cents on a few things that would have helped me out greatly to avoid the issues that i had. (Judging by the number of views that this thread has had i am guessing i am not the only one who has come up against these issues also)

Cheers
Sam