> the linux man pages for these functions literally don't mention this at all

> the fact that the man page never makes any note of this being an extension ([…]) is really notable to me.

> this is completely undocumented on the linux man page.

The Linux man page maintainers are really friendly in my experience, so it's probably worth reporting this to them [0] [1].

[0]: https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/...

[1]: https://lore.kernel.org/linux-man/