release instructions: new section about docs generation

RELEASING_RAILS.rdoc

@@ -205,3 +205,34 @@ There are two simple steps for fixing the CI:
 2. Fix it
 
 Repeat these steps until the CI is green.
+
+=== Manually trigger docs generation
+
+We have a post-receive hook in GitHub that calls the docs server on pushes.
+Triggers generation and publication of edge docs, updates the contrib app,
+and generates and publishes stable docs if a new stable tag is detected.
+
+The hook unfortunately is not invoked by tag pushing, so once the new stable
+tag has been pushed to origin, please run
+
+    curl -X POST -d '' http://rails-hooks.hashref.com/rails-master-hook
+
+You should see something like this:
+
+    Rails master hook tasks scheduled:
+
+      * updates the local checkout
+      * updates Rails Contributors
+      * generates and publishes edge docs
+
+    If a new stable tag is detected it also
+
+      * generates and publishes stable docs
+
+    This needs typically a few minutes.
+
+Note you do not need to specify the tag, the docs server figures it out.
+
+Also, don't worry if you call that multiple times or the hook is triggered
+again by some immediate regular push, if the scripts are running new calls
+are just queued (in a queue of size 1).