Document the log_cache_events config (#4821)

[Preview](https://docs-getdbt-com-git-dbeatty-log-cache-events-config-dbt-labs.vercel.app/reference/global-configs/logs#logging-relational-cache-events)

resolves #4664

## What are you changing in this pull request and why?

As noted in #4366, there is currently not any documentation for the
`log_cache_events` config.

When `True`, this flag enables verbose logging for [relational
cache](https://docs.getdbt.com/reference/global-configs/cache) events
(which can be helpful when debugging) (default to `False`).

Because it has existed since at least
[2018-11-15](dbt-labs/dbt-core#1139) (prior to
dbt v1.0), no version blocks are applicable.

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.

website/docs/reference/global-configs/logs.md

@@ -121,6 +121,14 @@ dbt --quiet run
 
 ```
 
+### Logging relational cache events
+
+The `LOG_CACHE_EVENTS` config allows detailed logging for [relational cache](reference/global-configs/cache) events, which are disabled by default.
+
+```text
+dbt --log-cache-events compile
+```
+
 <VersionBlock firstVersion="1.5">
 
 ### Color