volinfo: fix documenting comments

As pointed by Andrew deason during review of the 1.6 pullup of
commit ae27283550dab33704f30e18975722e0ed2c5424, psize is not
a parameter of HandleHeaderFiles, and in function HandleSpecialFile
it is of type inout since the value is first read by the += operation.
Fix this, and try to improve the description of psize too.

Change-Id: Ia728b20475f0c44b6104dc954aaa04d5f0f098b5
Reviewed-on: http://gerrit.openafs.org/11319
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: D Brashear <shadow@your-file-system.com>