Add documentation for the new Checks JavaScript Plugin API

Change-Id: I82cd3c9af9c918b1c26ea1cb8a70e3283138ac31

Documentation/pg-plugin-dev.txt

diff --git a/Documentation/pg-plugin-dev.txt b/Documentation/pg-plugin-dev.txt
index bcb59d2..dc7986f 100644
--- a/Documentation/pg-plugin-dev.txt
+++ b/Documentation/pg-plugin-dev.txt
@@ -375,6 +375,12 @@
 Returns an instance of the
 link:https://gerrit.googlesource.com/gerrit/+/master/polygerrit-ui/app/api/change-reply.ts[ChangeReplyPluginApi].
 
+[[checks]]
+=== checks
+`plugin.checks()`
+
+Returns an instance of the link:pg-plugin-checks-api.html[ChecksApi].
+
 === getPluginName
 `plugin.getPluginName()`