[pigeon] doc comments always start with ' '

[tarrinneal]

adds a space to all documentation comments that do not start with a space to avoid issues with languages that use */ to close block comments.

List which issues are fixed by this PR. You must list at least one issue.
fixes flutter/flutter#115657

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist
I read the Contributor Guide and followed the process outlined there for submitting PRs.
I read the Tree Hygiene wiki page, which explains my responsibilities.
I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
I signed the CLA.
The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
I listed at least one issue that this PR fixes in the description above.
I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
I updated/added relevant documentation (doc comments with ///).
I added new tests to check the change I am making, or this PR is test-exempt.
All existing and new tests are passing.

[gaaclarke]

LGTM. As a nit I think you could have gotten away with just testing the function you edited directly instead of at all the points in the generators. I think it would be cleaner and easier to maintain since the change would have a specific test associated with it.

[tarrinneal]

LGTM. As a nit I think you could have gotten away with just testing the function you edited directly instead of at all the points in the generators. I think it would be cleaner and easier to maintain since the change would have a specific test associated with it.

Good point, I just thought it made sense to add it with the other doc comment tests. I'll look into that change in the future.

I also thought this was closer to integration style testing.