MFC Capsicum open(2) and openat(2) documentation

r306537 by cem: open.2: Document Capsicum behavior

Document open(2) and openat(2) behavior in Capsicum capability mode.

Sponsored by: Dell EMC Isilon

r323622 by emaste: open(2): update ENOTCAPABLE description for .. lookups

After r308732 (MFC of r308212) Capsicum permits .. lookups in capability
mode, as long as path component traversal does not escape the directory
corresponding to the provided file descriptor.

Sponsored by: The FreeBSD Foundation