veyron: making sure that files create by 'go generate' files are up-to-date

Change-Id: I314fadd7298a6d18268087d3cb491bae7e001ead
diff --git a/tools/build/doc.go b/tools/build/doc.go
index bd39613..540cf91 100644
--- a/tools/build/doc.go
+++ b/tools/build/doc.go
@@ -35,7 +35,10 @@
    local namespace root; can be repeated to provided multiple roots
  -veyron.vtrace.cache_size=1024
    The number of vtrace traces to store in memory.
- -veyron.vtrace.dump_on_shutdown=false
+ -veyron.vtrace.collect_regexp=
+   Spans and annotations that match this regular expression will trigger trace
+   collection.
+ -veyron.vtrace.dump_on_shutdown=true
    If true, dump all stored traces on runtime shutdown.
  -veyron.vtrace.sample_rate=0
    Rate (from 0.0 to 1.0) to sample vtrace traces.