Review ServerProperties and document keys that only work in certain environments

Currently ServerProperties contains quite a mix of things. We’ll need to refactor it to get cleaner reactive/servlet concerns.

[bclozel]

Looking at the current properties arrangement, it seems that:

• server.display-name is only applied to Servlet environments, and should move to the server.servlet namespace
• server.tomcat.redirect-context-root, server.tomcat.use-relative-redirects and server.tomcat.additional-tld-skip-patterns are only applied to Servlet environments; I don’t think we really need to move those under a server.tomcat.servlet namespace…

[snicoll]

We’ve reviewed this issue and we’ve explored a few options that would guide users towards which environment is supported for a given key. Unfortunately, none of those options are satisfactory. It really feels like #4818 and we believe documenting the keys that are limited to certain environments is a more pragmatic approach to this issue.

This can be a simple extension of the key description with a standard message for now. Perhaps doing this work will give us improvement ideas for the metadata format that we can implement in a future feature release.