Added explanatory comment about primary key in oligo_probe.
Added explanatory comment about primary key in oligo_probe.

danstaines/ensembl,willmclaren/ensembl,at7/ensembl,mjg17/ensembl,muffato/ensembl,at7/ensembl,muffato/ensembl,Ensembl/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,muffato/ensembl,Ensembl/ensembl,at7/ensembl,danstaines/ensembl,willmclaren/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,muffato/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,at7/ensembl,mjg17/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl