Rename build target to release-candidate-checklist
Add sanity check for helpdb.jl

The awk one-liner provides a heuristic for checking if Sphinx added
those terrible extra newlines and prints out a warning if they are
present.
release-candidate-checklist now depends on release

Also reorder documentation build so that helpdb.jl comes last. Should be
easier to see if the helpdb build was bad and printed a warning.