viced: Clarify comment explaining cba sorting

The current comment here is very brief; it may not be immediately
clear to a reader why we are sorting these, and so why we need the
given CBAs in an array. Expand on it a bit.

Note that it seems like it might be possible to refactor multi_Rx to
not require all calls to be created before any packets are sent. If
multi_Rx were changed to send data as we create calls, it may be
possible to eliminate this sorting, and allow for slightly more
efficient callback traversal when breaking callbacks.

Change-Id: I966cdcf6d40aa5c02f8768f4dd76c580c811ccaf
Reviewed-on: http://gerrit.openafs.org/10170
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>