Update bcftools.txt

Petr, I'm not sure, if you want examples in the documentation or your HOWTO document. I added to the existing examples. I can do tabix -p bed file.bed.gz, but I don't know how to tabix index a tab delimited file. Valid values according to the documentation are gff, bed, sam, vcf and psltab. Therefore I added a bed annotation example showing that POS should just be replaced by FROM,TO. This was not obvious to me.