Various updates to the ftruncate(2) documentation:

Various updates to the ftruncate(2) documentation:
- Note that ftruncate(2) can operate on shared memory objects and cross
  reference shm_open(2).
- Note that ftruncate(2) does not change the file position pointer (aka
  seek pointer) of the file descriptor.
- ftruncate(2) will fail with EINVAL for all sorts of other fd types than
  just sockets, so instead note that it fails for all but regular files and
  shared memory objects.
- Note that ftruncate(2) also appeared in 4.2BSD along with truncate(2).
  (Or at least the manpage for both appeared in 4.2, I did not check the
  kernel code itself to see if either predated 4.2.)

PR:		199472 (2)
Submitted by:	andrew@ugh.net.au (2)
MFC after:	1 week