[clang-doc] Add tests for missing clang-doc output

[ilovepi]

The tests in clang-doc do not cover many cases for functions, templates, concepts, and many more C++ constructs.

Functions/methods do not test:

• noexpect
• throw()
• const
• constexpr
• consteval
• ...

Templates do not test constraints like the following:

template <class T = void>
class C {};

We should improve testing for functions and templates in the cases that we know about.