When determining whether or not to fail with `--fail-on-change`, only
look at .md files. In some cases, running the necessary commands (e.g.,
`flutter pub get`) may change unrelated files, causing fales positive
failures. Only changed documentation files should be flagged.
Also log the specific files that were detected as changed, to aid in
debugging any future false positives.
Fixes https://github.com/flutter/flutter/issues/111592
Fixes https://github.com/flutter/flutter/issues/111590
