Update styling of files documenting rules

[felicio]

Increase legibility of negative sentences in counter examples.

[ljharb]

Seems reasonable. Should this change be made consistently in every .md file?

[felicio]

I'd definitely agree. At first it might seem as a tooth-picking request, but when I was creating this PR I thought about that the docs are not always read without stress and little things like that could save someone from extra frustration.

The pattern here seems to be in 50+ markdown files where negative verbs (e.g. is/are not) within a paragraph perceived by and followed by a code block should be bold.

If no one jumps in with some clever sed or awk implementations, I could have a look at the rest myself.

[felicio]

I've just squashed and updated the branch with force. Shouldn't it trigger another review request? Nevertheless, I labeled this one as WIP, since it's missing the CONTRIBUTING.md.

[ljharb]

Thanks!