Improvement identified in #2790.
Add a prefix to the diff-id when it is used multiple times in different
"how to fix it in XYZ" sections to avoid ambiguity and pedantically
follow the spec:
> A single and unique diff-id should be used only once for each type of
code example as shown in the description of a rule.
Obvious typos around `diff-type` were fixed.
An obvious extra use of diff blocks was removed.
When an include is not surrounded by empty lines, its content is inlined
on the same line as the adjacent content. That can lead to broken tags
and other display issues.
This PR fixes all such includes and introduces a validation step that
forbids introducing the same problem again.
