Fixed an example of using scope_exit with std::function in the docs.

Closes boostorg#20.

Fixed an example of using scope_exit with std::function in the docs.

Closes boostorg#20.

Improved docs for unallocated_resource.

Expanded on the requirements for resource types and values supported by
unallocated_resource.

Added a note in the INVALID_HANDLE_VALUE example that says it relies on
MSVC extension and other compilers may not accept it.

Improved docs for unallocated_resource.

Expanded on the requirements for resource types and values supported by
unallocated_resource.

Added a note in the INVALID_HANDLE_VALUE example that says it relies on
MSVC extension and other compilers may not accept it.

Switch clang-12 through 15 to libstdc++11.