fix typo in DictDB.size_limit

limit meant to be 1GB, but typo made it 1MB
add DictDB to autogenerated config / help

for cull fraction, etc.
Merge pull request #2997 from minrk/db_limit

fix DictDB default size limit

typo had the default size as 1MB, not the intended 1GB.
Also fix its exclusion from generated config.