libstdc++: Improve comments in std::string tests

libstdc++: Improve comments in std::string tests

The COW std::string does support some features of C++11 allocators, just
not propagation. Change some comments in the tests to be more precise
about that.

libstdc++-v3/ChangeLog:

	* testsuite/21_strings/basic_string/allocator/char/copy.cc: Make
	comment more precise about what isn't supported by COW strings.
	* testsuite/21_strings/basic_string/allocator/char/copy_assign.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/move.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/move_assign.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/noexcept.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/operator_plus.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/char/swap.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/copy.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/copy_assign.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/move.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/move_assign.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/noexcept.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/operator_plus.cc:
	Likewise.
	* testsuite/21_strings/basic_string/allocator/wchar_t/swap.cc:
	Likewise.