content/docs/operating/storage.md: Clarify GOGC (#731)

content/docs/operating/storage.md

@@ -41,7 +41,8 @@ Because Prometheus uses most of its heap for long-lived allocations of memory
 chunks, the
 [garbage collection target percentage](https://golang.org/pkg/runtime/debug/#SetGCPercent)
 is set to 40 by default. You can still override this setting via the `GOGC`
-environment variable as usual.
+environment variable as usual. If you need to conserve CPU capacity and can
+accept running with fewer memory chunks, try higher values.
 
 For high-performance set-ups, you might need to adjust more flags. Please read
 through the sections below for details.