salvager: Improve comments

"these used to be asserts" is not a useful comment. This area of code
does maybe look a little confusing at first glance, though, so replace
these with comments that are more informative.

Change-Id: I4e0b9dff3d010931e02559e82165ffbd61c5b189
Reviewed-on: http://gerrit.openafs.org/10313
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>