Docs were not clear about the callback. Improved
Docs were not clear about the callback. Improved

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