## What changes were proposed in this pull request?

The discussion of the interaction of Accumulators and Broadcast Variables should logically follow the discussion on Checkpointing. As currently written, this section discusses Checkpointing before it is formally introduced. To remedy this:

 - Rename this section to "Accumulators, Broadcast Variables, and Checkpoints", and
 - Move this section after "Checkpointing".

## How was this patch tested?

Testing: ran

$ SKIP_API=1 jekyll build

, and verified changes in a Web browser pointed at docs/_site/index.html.

Author: José Hiram Soltren <jose@cloudera.com>

Closes #15281 from jsoltren/doc-changes.