[boost] [ptr_container] Docs not updated to reflect std::auto_ptr -> std:unique_ptr changes

An issue was recently created that while ptr_container now supports std::unique_ptr where appropriate and no longer supports std::auto_ptr where it has been removed from the C++ standard, the docs were never updated to reflect this change. I thought it would be worthwhile mentioning this as we go to the 1.67 release. A contributo is preparing a PR to correct this issue, and I will review as a member of CMT once the PR is submitted. I don't mean this as a way to delay the new release but I thought I should bring this up in the mailing list so others know this is the case.