build: comment possible ulimit failure in run.bash

Record what's going on in case someone is debugging a failure there.
It's not Go's fault.

Fixes #7381.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/68200043

src/run.bash

@@ -17,6 +17,12 @@ ulimit -c 0
 # Raise soft limits to hard limits for NetBSD/OpenBSD.
 # We need at least 256 files and ~300 MB of bss.
 # On OS X ulimit -S -n rejects 'unlimited'.
+#
+# Note that ulimit -S -n may fail if ulimit -H -n is set higher than a
+# non-root process is allowed to set the high limit.
+# This is a system misconfiguration and should be fixed on the
+# broken system, not "fixed" by ignoring the failure here.
+# See longer discussion on golang.org/issue/7381. 
 [ "$(ulimit -H -n)" == "unlimited" ] || ulimit -S -n $(ulimit -H -n)
 [ "$(ulimit -H -d)" == "unlimited" ] || ulimit -S -d $(ulimit -H -d)