Expand comment about how StringsToBuckets was computed, and add more entries

The construction was explained in
https://reviews.llvm.org/D44810?id=139526#inline-391999 but reading the code
shouldn't require hunting down old reviews to understand it.

The precomputed list was missing an entry for the empty list case, and
one entry at the very end. (The current last entry is the last one where
3 * BucketCount fits in a signed int, but the reference implementation
uses unsigneds as far as I can tell, so there's room for one more entry.)

No behavior change for inputs seen in practice.

Differential Revision: https://reviews.llvm.org/D64738

llvm-svn: 366107