VPP-279: Document changes for vnet/vnet/devices

VPP-279: Document changes for vnet/vnet/devices

Add doxygen documentation for pcap tx trace CLI command.

In the process of adding the documentation, made the following changes
to the way the command worked:
* If there is an error with any of the attributes, the whole command
  fails. The existing behavior was to apply attribute by attribute,
  then bail if there was an issue, with partial apply.
* Move the 'on' processing to the end. The existing behavior was to
  process the 'on' as it was encountered on the commandline. That meant
  that any attributes after the 'on' in the commandline were saved and
  displayed, but not really being used in the packet trace.
* Enhanced the 'status' to show all the configured attributes.

NOTE: The packet capture has some weird behavior with regards to how
many packets are written to file and if the file is appended or
overwritten. VPP-634 written to document the issue.

Change-Id: Iab241228b125385052de242865afd9515fa2524f
Signed-off-by: Billy McFall <bmcfall@redhat.com>