pgsql: Update comments about progress reporting by index_drop

Update comments about progress reporting by index_drop Michaël Paquier complained that index_drop is requesting progress reporting for non-obvious reasons, so let's add a comment to explain why. Discussion: https://postgr.es/m/[email protected] Branch -- REL_12_STABLE Detail

pgsql: Update comments about progress reporting by index_drop

Update comments about progress reporting by index_drop Michaël Paquier complained that index_drop is requesting progress reporting for non-obvious reasons, so let's add a comment to explain why. Discussion: https://postgr.es/m/[email protected] Branch -- master Details -