Minor archive docs feedback

[bhsiaoNFLX]

If the state conditional is:

function archive (schema) {
  ...
  state < '${states.active}' AND keepUntil < now()
  ...

I.e. configuration.md should really be:

"...created and retry state before it's archived"

Am I right?

Also, archiveCompletedAfterSeconds is currently missing documentation even though there's a unit test for it. It should be documented.

If you agree, happy to create a PR :)

[timgit]

Yes. Please open a PR. Thanks!

[bhsiaoNFLX]

PR: https://github.com/timgit/pg-boss/pull/221/commits