Tidy up dev-plugins.txt documentation

In I7742837f2b "post review" link in "Post Review Extensions" was
misconfigured.

In I5869c8168f "Actions" section was erroneously removed.

Change-Id: Ia3b09d5febcfe9ab1805bd2e55963aa6c6624f6a

Documentation/dev-plugins.txt

diff --git a/Documentation/dev-plugins.txt b/Documentation/dev-plugins.txt
index f2a3e12..a66d3b5 100644
--- a/Documentation/dev-plugins.txt
+++ b/Documentation/dev-plugins.txt
@@ -1450,7 +1450,7 @@
 
 By implementing the `com.google.gerrit.server.restapi.change.OnPostReview`
 interface plugins can extend the change message that is being posted when the
-[post review](rest-api-changes.html#set-review) REST endpoint is invoked.
+link:rest-api-changes.html#set-review[post review] REST endpoint is invoked.
 
 This is useful if certain approvals have a special meaning (e.g. custom logic
 that is implemented in Prolog submit rules, signal for triggering an action
@@ -1458,6 +1458,8 @@
 in the change message. This makes the effect of a given approval more
 transparent to the user.
 
+[[ui_extension]]
+== UI Extension
 
 [[actions]]
 === Actions