There has been added information how to use buffer-request handler by JBEAP-14253. The CLI command example given has not perfect formatting as it might not be obvious that those are actually two commands as first one is wrapped and no new-empty-line is added between those two:

/subsystem=undertow/configuration=filter/expression-
filter=buf:add(expression="buffer-request(buffers=1)")
/subsystem=undertow/server=default-server/host=default-host/filter-ref=buf:add

Please, either remove line breaking and/or put an empty new-line between those two command examples.