DOCS-8259 relnotes part

Author: kay-kim

source/release-notes/3.4.txt

@@ -116,7 +116,15 @@ acknowledgement for a write concern of :writeconcern:`majority
 <"majority">` returns after the majority of the voting members apply
 the write in memory or to the on-disk journal *if* the :ref:`j <wc-j>`
 option is unspecified in the write concern.
-   
+
+Adjustable Catchup Period for Newly Elected Primary
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A new replica set configuration setting
+:rsconf:`settings.catchUpTimeoutMillis` defines the time limit for a
+newly elected primary to catch up with the other replica set members
+that may have more recent writes.
+
 .. _3.4-decimal:
 
 Decimal Type