lseek(2): Document the seek behavior better and update the POSIX compliance

In certain situations lseek(2) will return successful although if no seek
was performed. This can happen when operating on devices that don't support
seeking (older tape drives) or when operating on changeable media devices
(such as DVD or Blu-ray devices) without a medium inserted.

Document this within the man page and update the POSIX compliance while here.

PR: 162765
Submitted by: arundel@
Reported by: arundel@
Reviewed by: bcr (mentor)
Approved by: bcr (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25646