Document better transitive_headers=True

[memsharded]

          @memsharded today i fell into this pitfall myself and wasted hours debugging it.

"transitive_headers" isn't mentioned in the tutorials (only in the reference) and conan new's generated conanfile doesn't include it.
users won't know it exists and could assume Conan is broken.

I think it should be mentioned in:
https://docs.conan.io/2/tutorial/creating_packages.html
ideally in a way that's hard to miss in case users skim it.

or better yet default to something that just works™, seems like a premature optimization.
though i understand this decision was already made.

Originally posted by @Enhex in conan-io/conan#17318 (comment)