message: update API notes
diff: /*
  - Indexer deals with indexing documents. It's public API is just
  - `update`, which is responsible for getting any new upstream content
  - into the search index.
  + Indexer deals with indexing documents. Its public API is
  - update takes these optional arguments:
  + `update`: responsible for getting any new upstream content into
  + the search index. update takes these optional arguments:
  - realTime: when true, update will block until the resulting
  changes are visible in elasticsearch. This is somewhat
  the hint can make it easier to keep the search index nearly
  real-time fresh.
  + `schemaForBranch(branch)`: retrieves the Schema for a given
  + branch. A Schema instance is computed from all the schema models
  + that are discovered on a branch. Schema models are things like
  + `content-types`, `fields`, `data-sources`, `plugin-configs`,
  + etc. They are pieces of content, but special pieces of content
  + that can alter how other content gets indexed.
  +
  + This method does it own caching, since schemas get computed as
  + part of indexing anyway. You can also directly invalidate the
  + cache, see next method.
  +
  + `invalidateSchemaCache()`: does what it says on the
  + tin. This is a lighter-weight operation than `update`. It allows
  + us to decouple the question of when and how to index content
  + from the issue of maintaining schema correctness during
  + sequences of writes.
  +
  */
  const log = require('@cardstack/plugin-utils/logger')('indexers');
  