What man page(s) discuss that when a file is created it inherits the group 
ID of its directory (instead of the gid of the creating process)?

Linux doesn't do that by default and calls it "BSD semantics". Linux can 
enable that with S_ISGID bit.

Can it be disabled on a BSD system? In other words on NetBSD, how to make 
it so new files are automatically created with the group of creating 
process (instead of the group of the directory)? I am just curious and I 
don't need this.

  Jeremy C. Reed