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.
