Add a "DO NOT EDIT" comment to failpoint-enabled source code

[kennytm]

Feature Request

Is your feature request related to a problem? Please describe:

Although failpoint-ctl disable will automatically apply diff when the original source is changed, this doesn't always work, and it would be frustrating to be stuck in a state where failpoint-ctl disable cannot proceed and you've got some changes hard to be extracted.

Most editors supporting Go recognize the generated source indicator and warns when you try to edit the file, thus preventing the need to merge diffs in the first place.

Describe the feature you'd like:

In the source code generated by failpoint-ctl enable (all of the rewritten *.go, *__failpoint_binding__.go, and *.go__failpoint_stash__), insert this comment on first line:

// Code generated by failpoint DO NOT EDIT.

Remove this line after failpoint-ctl disable.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Optimization:

[lonng]

Can you file a PR for this feature? @kennytm

[kennytm]

Yeah when I've got time to do it 🙃