[clang-doc] Initialize StringRef members in Info types (#191637)
We had a report of some assertion failures in
https://github.com/llvm/llvm-project/pull/190054#issuecomment-4228893309,
and some msan failures in
https://github.com/llvm/llvm-project/pull/190056.
These appear to be due to default constructed StringRef's being used in
some cases. To address, we can provide default initializers that should
prevent such cases from causing further problems.