doc: Add release notes for getindexinfo RPC

Author: fjahr

doc/release-notes.md

@@ -115,6 +115,10 @@ Changes to Wallet or GUI related RPCs can be found in the GUI or Wallet section
 New RPCs
 --------
 
+- The `getindexinfo` RPC returns the actively running indices of the node,
+  including their current sync status and height. It also accepts an `index_name`
+  to specify returning only the status of that index. (#19550)
+
 Build System
 ------------