**what is the change?:**
This just updated the error codes on the docs.

For my future self, recording the steps it took to get here:
- On master, run `npm run build -- --extract-errors`
- Create a commit with those changes, and open a PR
  (react#10619)
- Cherry-pick that commit onto 15-stable, and open a PR
  (react#10621)
- Update the error codes on the `gh-pages branch` by doing the
  following based on docs
  (https://github.com/facebook/react/tree/master/docs#updating-facebookgithubioreact):
  - Set up a sister directory to `react` with `gh-pages` checked out,
    and call it `react-gh-pages`
  - In your `react` repo, with `15-stable` checked out and the
    error-codes update cherry-picked, run:
    `cd docs && bundle rake exec && bundle exec rake fetch_remotes && bundle exec rake release`
  - `cd ../react-gh-pages` and check in those changes, create a
    commit/PR
  - Manually test running the docs

**why make this change?:**
Even though this is just an RC, and not a final major release, we still
want the docs to be as up-to-date as possible.

**test plan:**
Run the docs locally and make sure things work.
(Flarnie will insert screenshots.)

**issue:**
react#8854