Add elastic#46346 to 7.9 release notes (elastic#68073)

In elastic#46346 we changed the distribution of work across event loops but
this was marked as a `>non-issue` so did not get a mention in the
release notes. However, plugin authors might need to be aware of this
change, so this commit records it in the release notes as an enhancement
instead.

Closes elastic#67960

Author: DaveCTurner

docs/reference/release-notes/7.9.asciidoc

@@ -561,6 +561,7 @@ Mapping::
 
 Network::
 * Make the number of transport threads equal to the number of available CPUs {es-pull}56488[#56488]
+* Share Netty event loops between transports {es-pull}46346[#46346]
 
 Recovery::
 * Implement dangling indices API {es-pull}50920[#50920] (issue: {es-issue}48366[#48366])