libstdc++: Add more Doxygen comments and another test for std::out_ptr

libstdc++: Add more Doxygen comments and another test for std::out_ptr

Improve Doxygen comments for std::out_ptr etc. and add a test for the
feature test macro.  Also remove a redundant preprocessor condition.

Ideally the docs for std::out_ptr and std::inout_ptr would show examples
of how to use them and what they do, but that would take some effort.
I'll aim to do that before GCC 14 is released.

libstdc++-v3/ChangeLog:

	* include/bits/out_ptr.h: Add Doxygen comments. Remove a
	redundant preprocessor condition.
	* testsuite/20_util/smartptr.adapt/version.cc: New test.