If, rather than a documentation bug, you think you've found a bug in a system call (i.e., in the Linux kernel), or in a library function (i.e., in glibc), look here.
If you want to report a bug for a man page that belongs to a package other than man-pages, look here.
When making a bug report please:
The way to report a man-pages bug depends on the type of bug:
You will need to create an account on this bugzilla if you don't already have one. This bugzilla is oriented toward Linux kernel bugs, so includes template text that is not relevant for most man-pages bug reports. Don't worry about reporting the Tree, Alias, or Kernel Version (just specify "n/a"), and you can ignore the template text under Description.
The list of unresolved bugs in in the bugzilla can be seen here.