Configuration: Expect HA gerrit.serverIds to match

Document that fact and its related considerations. Minimally amend the
surrounding documentation so this change fits in.

Add such configuration to the dockerized testing setup. Use [1] from [2]
to generate that uniquely random test UUID.

[1] https://uuidgen.org/v/4
[2] https://en.wikipedia.org/wiki/Universally_unique_identifier#External_links

Bug: Issue 12653
Change-Id: Idf94ae2721efe3a3122a8d9714df98a0ca805e32
diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index c7196c6..1d1a02b 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md
@@ -57,9 +57,10 @@
 
 For the masters:
 
-* Configure database section in gerrit.config to use the shared database
+* Configure the database section in gerrit.config to use the shared database
 * Configure gerrit.basePath in gerrit.config to the shared repositories location
-* Install and configure @PLUGIN@ plugin
+* Configure gerrit.serverId in gerrit.config based on [config](config.md)'s introduction
+* Install and configure this @PLUGIN@ plugin [further](config.md) or based on below.
 
 Here is an example of the minimal @PLUGIN@.config:
 
diff --git a/src/main/resources/Documentation/config.md b/src/main/resources/Documentation/config.md
index e79cbdd..dd8e675 100644
--- a/src/main/resources/Documentation/config.md
+++ b/src/main/resources/Documentation/config.md
@@ -2,8 +2,14 @@
 @PLUGIN@ Configuration
 =========================
 
-The @PLUGIN@ plugin must be installed on all the instances and the following fields
-should be specified in `$site_path/etc/@PLUGIN@.config` file:
+The @PLUGIN@ plugin must be installed on all the instances. Each instance should
+be configured with the same [gerrit.serverId](https://gerrit-documentation.storage.googleapis.com/Documentation/3.0.0/config-gerrit.html#gerrit.serverId).
+If there are existing changes in [NoteDb](https://gerrit-documentation.storage.googleapis.com/Documentation/3.0.0/note-db.html)
+made with another `serverId`, then this plugin might not be able to access them.
+Likewise, if the HA gerrit.serverIds differ, then changes conveyed by one
+instance will not be accessible by the other.
+
+The following fields should be specified in `$site_path/etc/@PLUGIN@.config` files:
 
 File '@PLUGIN@.config'
 --------------------
diff --git a/src/test/docker/etc/gerrit.config b/src/test/docker/etc/gerrit.config
index a4728b6..90a4057 100644
--- a/src/test/docker/etc/gerrit.config
+++ b/src/test/docker/etc/gerrit.config
@@ -1,6 +1,7 @@
 [gerrit]
 	basePath = git
 	canonicalWebUrl = http://gerrit:8080/
+	serverId = f7696647-8efd-41b1-bd60-d321bc071ea9
 [index]
 	type = LUCENE
 [auth]