core: add search in commands history in release notes (issue #2040)

ReleaseNotes.adoc

@@ -14,6 +14,25 @@ For a complete list of changes, please look at ChangeLog.
 [[v4.2.0]]
 == Version 4.2.0 (under dev)
 
+[[4.2.0_search_commands_history]]
+=== Search in commands history
+
+Search in commands history has been added with new keys and a new key context
+called "histsearch".
+
+Some existing keys have been moved as well.
+
+New keys can be changed and added with the following commands after upgrade
+from an old WeeChat version:
+
+----
+/key missing
+/key unbind ctrl-s,ctrl-u
+/key bind meta-U /allbuf /buffer set unread
+/key bind ctrl-r /input search_history
+/key bindctxt search ctrl-r /input search_previous
+----
+
 [[4.2.0_irc_rgb_colors]]
 === RGB colors in IRC messages