Jakub,should the man page be changed, and in what what?

nice(2) shouldn't be changed, aside from possibly mentioning that glibc-2.2.5
and later returns the new priority... the syscall is the same, AFAIK.

There should probably be a nice(3) that documents that the library interface
conforms to SuS. Or do we not do separate section 3 pages for places where glibc
changes the returns of syscalls?

In any case, especially if the symbol isn't versioned, we need to add this to
the release notes.

I don't think the (2) man pages describe the raw system calls. That is you can't
apply
the xxx(2) mage page to syscall (SYS_xxxx, ....).  For example,

STAT(2)                    System calls                   STAT(2)

NAME
       stat, fstat, lstat - get file status

SYNOPSIS
       #include <sys/types.h>
       #include <sys/stat.h>
       #include <unistd.h>

       int stat(const char *file_name, struct stat *buf);
       int fstat(int filedes, struct stat *buf);
       int lstat(const char *file_name, struct stat *buf);

DESCRIPTION
       These  functions  return  information  about the specified
       file.  You do not need any access rights to  the  file  to
       get  this  information  but  you need search rights to all
       directories named in the path leading to the file.

       stat stats the file pointed to by file_name and  fills  in
       buf.

You can't replace 

stat ("foo", &buf)

with

syscall (SYS_stat, "foo", &buf)

The xxx (2) man pages are for the cooked syscalls provided by glibc. You can
have a separate man page for syscall (). But I don't think adding nice (3) will
help anyone if there is nice (2).

The man page should document the standard. A deviation from that is a bug in the
component implementing it, not in the man page. If there is one, please add a
patch :)