rights.4: various corrections on capability rights

• A file descriptor obtained from accept(2), accept4(2) and openat(2) is not always assigned all capability rights. Instead, it inherits capability rights from the "parent" socket/dir file descriptor.
• getdents(2) and getdirentries(2) requires CAP_READ.
• openat(2) with O_WRONLY|O_TRUNC does not require CAP_SEEK.

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1207