Clarify healthcheck registration for lib installation

Document that, when the pull-replication is installed as a lib module,
the healthcheck plugin needs to be made available as module too, in
order for the healthcheck registration to work.

Change-Id: Iecc2adfb61bc91ac490f116db80c7a87b4843743

src/main/resources/Documentation/healthcheck.md

diff --git a/src/main/resources/Documentation/healthcheck.md b/src/main/resources/Documentation/healthcheck.md
index afe773b..3a42e6a 100644
--- a/src/main/resources/Documentation/healthcheck.md
+++ b/src/main/resources/Documentation/healthcheck.md
@@ -63,6 +63,11 @@
 is installed. If the `healthcheck` plugin is not installed, then the
 check registration is skipped during load of the pull-replication
 plugin.
+
+> **Note** when the @@PLUGIN@@ plugin is installed as a lib (see [extension-point.md](extension-point.md)),
+> then the healthcheck plugin jar should also be present in the `lib` directory,
+> for the check registration to work.
+
 - Because the pull-replication healthcheck depends on the `healthcheck` plugin, renaming/removing the `healthcheck`
 jar file is not supported during runtime. Doing so can lead to unpredictable behaviour of your gerrit instance.