lib/libcam/cam.3: note that cam_freeccb(3) with ccb == NULL is a no-op

This allows me to accurately test this scenario, and for others to rely
on the behavior, instead of relying on knowledge obtained via code
inspection.

Wording borrowed from free(3).

MFC after: 1 week
Requested by: ken (D9928)
Reviewed by: imp
Sponsored by: Dell EMC Isilon
Differential Revision: D9969