I think the man pages that say "should" may want to clarify that issue a little better; however, it does appear to have the correct information.
If you use Linux, the Linux documentation should be authoritative. Hopefully, it will agree with POSIX and C99 (or whatever is the latest memcpy standard) as much as possible. If there is a reason for a change (or to document a Linux bug) and you use Linux, I would pay attention to the Linux documentation and treat everything else as advisory. If you use Red Hat or whatever other distro, I would look treat those docs as authoritative and not whatever other standard you think should apply.
A different matter is arguing about keeping Linux in sync with POSIX, etc, but if you want to build software that will work, short of maintaining your personal set of patches not accepted by upstream, you would probably want to code to "Linux" (at least for the Linux port).