Suggestion: consolidate self-signed cert discussion

[laurencelundblade]

There is text warning not to trust self-signed certs in the four different sections. If it is in x5bag and x5chain it should be mentioned in x5t and x5u too, which would put in six different sections.

I think it would be more and efficient to have it just in security considerations, just in one place.

This is just a suggestion for clarity and efficiency, not that any thing is wrong with the current text.