Docs - clarify performance issues from ConstrainRAMSpace.

Revert some of my b50f4661. Elaborate on tradeoffs, and point
to HTC page as well which is a better location for this info.

doc/man/man5/cgroup.conf.5

@@ -114,6 +114,11 @@ which case the job's RAM limit will be set to its swap space limit if
 \fBConstrainSwapSpace\fR is set to "yes".
 Also see \fBAllowedSwapSpace\fR, \fBAllowedRAMSpace\fR and
 \fBConstrainSwapSpace\fR.
+NOTE: When enabled, ConstrainRAMSpace can lead to a noticiable decline in
+per-node job throughout. Sites with high-throughput requirements should
+carefully weigh the tradeoff between per-node throughput, versus potential
+problems that can arise from unconstrained memory usage on the node. See
+<https://slurm.schedmd.com/high_throughput.html> for further discussion.
 
 .TP
 \fBConstrainSwapSpace\fR=<yes|no>