Add comment to info/refs HTTP RPC

This comment is added as it takes a few times reading this before it
really sinks in. Hope that this helps others with this realization too,
and speed up their learning.
4 jobs for zj-comment-info-refs in 1 minute and 45 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #241454706
build

00:01:36

passed #241454707
check-grpc-proto-clients

00:01:33

passed #241454708
danger-review

00:00:29

passed #241454705
test

00:01:45