Add caveat for version ranges

[karlhorky]

Description:

Add warning and caveat for NVM syntax - to document the surprising behavior that it will not use the latest lts/* version for days, weeks or months after a new version is released.

Related issue:

Issue reported in https://github.com/actions/setup-node/issues/1236 and then closed, as a "won't fix"

Check list:

• [ ] Mark if documentation changes are required.
• [ ] Mark if tests were added or updated to cover the changes.