Describe most important search updates in 2.9 release notes

Change-Id: Ieb116bfd11ac7dab0226718c0092fc00a51c7806
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/user-search.txt

@@ -280,6 +280,7 @@ is:submitted, is:merged, is:abandoned::
 +
 Same as <<status,status:'STATE'>>.
 
+[[mergeable]]
 is:mergeable::
 +
 True if the change has no merge conflicts and could be merged into its

ReleaseNotes/ReleaseNotes-2.9.txt

@@ -28,6 +28,9 @@ Schema Change
   java -jar gerrit.war reindex --recheck-mergeable -d site_path
 ----
 
+*WARNING* Support for query via the SQL index is removed. The usage of
+a secondary index is now mandatory.
+
 *WARNING:* Upgrading to 2.9.x requires the server be first upgraded to 2.1.7 (or
 a later 2.1.x version), and then to 2.9.x.  If you are upgrading from 2.2.x.x or
 later, you may ignore this warning and upgrade directly to 2.9.x.
@@ -130,6 +133,44 @@ Now if `auth.gitBasicAuth` is set to `true` users that exist only in
 Gerrit but not in LDAP are authenticated with their HTTP password from
 the Gerrit database.
 
+Search
+~~~~~~
+
+* Support for query via the SQL index is removed. The usage of
+a secondary index is now mandatory.
+
+* New link:https://gerrit-documentation.storage.googleapis.com/Documentation/2.9/user-search.html#mergeable[
+is:mergeable] search operator
++
+Finds changes that have no merge conflicts and can be merged into the
+destination branch.
+
+* New link:https://gerrit-documentation.storage.googleapis.com/Documentation/2.9/user-search.html#parentproject[
+parentproject] search operator
++
+Finds changes in the specified project or in one of its child projects.
+
+* New link:https://gerrit-documentation.storage.googleapis.com/Documentation/2.9/user-search.html#conflicts[
+conflicts] search operator
++
+Finds changes that conflict with the specified change.
+
+* New operators for absolute last-updated-on search
+** link:https://gerrit-documentation.storage.googleapis.com/Documentation/2.9/user-search.html#before_until[
+before / until]
+** link:https://gerrit-documentation.storage.googleapis.com/Documentation/2.9/user-search.html#after_since[
+after / since]
+
+* Support exact match on file parts in
+link:https://gerrit-documentation.storage.googleapis.com/Documentation/2.9/user-search.html#file[
+file] operator
+
+* Query shortcuts
+** `o` = `owner`
+** `r` = `reviewer`
+** `p` = `project`
+** `f` = `file`
+
 Daemon
 ~~~~~~