Specify cache.threads = 0 in the multi-site config documentation

The multi-site cache propagation mechanism relies on the synchronous
notification of cache evictions callbacks from Caffeine, otherwise
it would result in notification loops between sites.

Add the cache.threads = 0 documentation setting so that the admin
is warned about it.

Change-Id: I52d52c76572654884e7bdae5e44b4b3f3ba30aa0

src/main/resources/Documentation/config.md

diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index a76545f..02806a1 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -9,7 +9,9 @@
 ## Configuration parameters
 
 ```cache.synchronize```
-:   Whether to synchronize cache evictions.
+:   Whether to run cache evictions synchronously. It requires disabling the
+    background cache evictions notifications in `gerrit.config` by setting
+    `cache.threads = 0`.
     Defaults to true.
 
 ```cache.threadPoolSize```