Add #46346 to 7.9 release notes (#68073)

In #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 #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])