A couple small improvements.

• Link to the up-to-date list of labels on github itself.
• Update some of the labels to match observed behavior of the usage of these labels. I could be wrong, though!
  • In the main repo, the documentation tag is not really used for documentation improvement requests (from users). Instead it is used for issues that are reported as bugs, but are actually limitations that can be (further) documented.
  • Similarly, the feature request label is no longer used much for feature request issues (since those are closed and turned into proposals), but it is used to tag PRs that do not have a corresponding proposal.
  • The needs work label is used sometimes for very incomplete bug reports, as opposed to the needs testing label for more complete reports that just need to be tested.