Minor documentation fix, see #164

Author: pd3

doc/bcftools.txt

@@ -3986,7 +3986,7 @@ they will evaluate to a vector of per-sample values when applied on FORMAT tags:
 
 * variables calculated on the fly if not present: number of alternate alleles;
 number of samples; count of alternate alleles; minor allele count (similar to
-AC but is always smaller than 0.5); frequency of alternate alleles (AF=AC/AN);
+AC but always picks the allele with frequency smaller than 0.5); frequency of alternate alleles (AF=AC/AN);
 frequency of minor alleles (MAF=MAC/AN); number of alleles in called genotypes;
 number of samples with missing genotype; fraction of samples with missing genotype;
 indel length (deletions negative, insertions positive, balanced substitutions zero)