Add documentation for possible causes of JS-related test failures.

What does this MR do?

• Adds a list of unsupported JS features to
• Explains how/why to enable JS in ruby specs
• Breaks up gotchas into separate sections, with more appropriate titles

Why is this MR needed?

I banged my head against this for quite a while today.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together