* Update settings endpoint modified
Now accepts index.routing.allocation.* settings but denies changing
the allocation setting that keeps watches on data nodes
* Get settings endpoint modified
Now returns index.routing.allocation.* settings explicitly filters out
the `index.routing.allocation.include._tier_preference` setting
* Tests for modified endpoints
* Update docs
**Problem:**
For historical reasons, source files for the Elasticsearch Guide's security, watcher, and Logstash API docs are housed in the `x-pack/docs` directory. This can confuse new contributors who expect Elasticsearch Guide docs to be located in `docs/reference`.
**Solution:**
- Move the security, watcher, and Logstash API doc source files to the `docs/reference` directory
- Update doc snippet tests to use security
Rel: https://github.com/elastic/platform-docs-team/issues/208