doc 32bit workspace_backend bump

Ref 8be771d1
Ref #2932

doc/sphinx/whats-new/changes-trunk.rst

@@ -77,6 +77,10 @@ the flag is turned on, C sources and shared object libraries that were
 generated from VCL sources are retained in the Varnish working
 directory (see the notes about ``varnishtest`` below).
 
+For 32bit Platforms, we have increased the default
+:ref:`ref_param_workspace_backend` from 16k to 20k accomodate larger
+response headers have become common.
+
 Other changes in varnishd
 ~~~~~~~~~~~~~~~~~~~~~~~~~