Adding reclaim_disowned logic & miscellaneous naming and documentation improvements. (#2943)

* Using new smart_holder::reclaim_disowned in smart_holder_type_caster for unique_ptr.

* Systematically renaming was_disowned to is_disowned (because disowning is now reversible: reclaim_disowned).

* Systematically renaming virtual_overrider_self_life_support to trampoline_self_life_support (to reuse existing terminology instead of introducing new one).

* Systematically renaming test_class_sh_with_alias to test_class_sh_trampoline_basic.

* Adding a Trampolines and std::unique_ptr section to README_smart_holder.rst.

* MSVC compatibility.