[clang-tools-extra] [clang-doc][nfc] Avoid constructing SmallString in ToString method (PR #96921)

[Petr Hosek via cfe-commits]

petrhosek wrote:

I'd use `std::string_view` as a return value instead of `const char *` as a more idiomatic C++ in which case you don't need to rename the method to `c_str()`.

https://github.com/llvm/llvm-project/pull/96921