standards.7 says:
The interfaces documented in POSIX.1/SUS are available as manual pages
under sections 0p (header files), 1p (commands), and 3p (functions);
thus one can write man 3p open.

However, under Debian these man pages are not shipped, so this does
not work.

So please ammend this as follows (or similar):

The interfaces documented in POSIX.1/SUS are available as manual pages
under sections 0p (header files), 1p (commands), and 3p (functions);
thus one can write man 3p open. Note that on Debian systems these man
pages are not locally installed due to license restrictions, use an
online version instead.

If you accept this idea and agree to ship a patch for this, I can
provide you with one.