doc: document runtimecontentionstacks

For #57071.

Change-Id: Ic1645af57aa589917c67154a5e4ad0b4edd7ba90
Reviewed-on: https://go-review.googlesource.com/c/go/+/547058
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Rhys Hiltner <rhys@justin.tv>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

Author: prattmic

doc/godebug.md

@@ -178,6 +178,13 @@ should adjust their Linux configuration according to the recommendations in the
 [GC guide](/doc/gc-guide#Linux_transparent_huge_pages), or switch to a Linux
 distribution that disables transparent huge pages altogether.
 
+Go 1.22 added contention on runtime-internal locks to the [`mutex`
+profile](/pkg/runtime/pprof#Profile). Contention on these locks is always
+reported at `runtime._LostContendedRuntimeLock`. Complete stack traces of
+runtime locks can be enabled with the [`runtimecontentionstacks`
+setting](/pkg/runtime#hdr-Environment_Variable). These stack traces have
+non-standard semantics, see setting documentation for details.
+
 ### Go 1.21
 
 Go 1.21 made it a run-time error to call `panic` with a nil interface value,