proplib documentation patch

["S K" <ekamperi%gmail.com@localhost>]

Hello everybody.

I finished my patch regarding proplib documentation enhancements:
http://stathisk.ath.cx/proplib-doc.diff

The changes I've made are:

* Added return values on failure in various prop_xxx man pages
  after auditing the respective source code
* Added comment regarding expansion step in prop_array_add()
* Fixed a typo in prop_data(3) and in prop_copyin_ioctl(9)
* Added new section RETURN VALUES in prop_send_ioctl(3)
  (there was no such nor the return values were documented on a
per-function basis)
* Added new section ERRORS in prop_send_ioctl(3)
* Added new section EXAMPLES in prop_send_ioctl(3) and moved
_existing_ code there
* Added new section RETURN VALUES in prop_copyin_ioctl(9)
  (there was no such nor the return values were documented on a
per-function basis)
* Added new section ERRORS in prop_copyin_ioctl(9)
* Added new section EXAMPLES in prop_copyin_ioctl(9) and moved
_existing_ code there

If anyone objects to some of the above, I can change it and send a new one.

Regards,
Stathis Kamperis