open(2): update ENOTCAPABLE description for .. lookups

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

We should add a description of the vfs.lookup_cap_dotdot and
vfs.lookup_cap_dotdot_nonlocal sysctls, perhaps as a cross-reference to
capsicum(4). I intend to look at that soon.

Reviewed by: bjk, cem, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12343