Fix docs, comment about FFS_AT start parameter

[dougm]

Summary

In its definition, and as it is used throughout freebsd code, the start parameter is a 0-based index into the bit array, not a 1-based index. Correct misleading comments and documentation.

[dougm]

Oh, I see. Your 'greater than a start bit' makes your decision to call this 1-based correct. Confusing to me, but correct. Sorry to bother you.