Update whats-new.adoc to mention attribute name change from `page-relative` to...

Update whats-new.adoc to mention attribute name change from `page-relative` to `page-relative-src-path`, with suggestion for bi-version compatibility.
3 jobs for djencks-v2.3.x-patch-70688 in 5 minutes and 11 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Init
passed #836402352
yarn

00:02:19

 
  Verify
passed #836402353
lint

00:01:10

passed #836402354
test

00:02:51