Help text is misleading about the behavior of --pdb

[SnoopJ]

The help string for --pdbcls currently reads "start a custom interactive Python debugger on errors", which is a bit misleading because the flag only does start a debugger if --pdb is passed. It looks like this was an accurate description when --pdbcls implied --pdb, but the help text wasn't updated when the flags were decoupled in #1952.