Further Improve Discussion of Secure Transport in Security Schemes

[mmccool]

In #1542, it's still unclear which schemes need TLS and which don't, and which are very weak without it (e.g. Basic). Suggest changing "Generally" in new para to "Except where noted" and then specifying in each schemes that does NOT need secure transport (e.g. in "nosec" add "This scheme does not require secure transport.", in "combo" add "This scheme does not itself require secure transport, but the schemes it combines may.")

[sebastiankb]

from today's TD call:

• Security TF will discuss this topic
• most likely there will be a PR which provide additional text about TLS. However, this input will be informative and not normative.