Closes #16942.

- Make `Descriptor` overview subtext of `Interface for parsed descriptor objects.`
- Conform to `@param[in, out] argname: Info` in parameter comments. Present in code: https://github.com/bitcoin/bitcoin/blob/feb162d500273040c8a2e5cdd3393edf34dff45b/src/net_processing.cpp#L1001
- Remove redundant argument type, `in` vs `out` mentions
- Removed unnecessary backticks around `IsSolvable()`, since Doxygen builds a link to the known function's docs
- Add backticks to refer to `argname`s

`descriptor.cpp` has more documentation, but Doxygen's output doesn't include anything inside unnamed namespaces for some reason. Tried to access them via searchbar.