time: document that going to sleep may stop the monotonic clock
time: document that going to sleep may stop the monotonic clock

Fixes #23178

Change-Id: I060a73d6263bc135f5a14c1991932a225208bb39
Reviewed-on: https://go-review.googlesource.com/103396
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,CAFxX/go,akutz/go,nawawi/go,akutz/go,nawawi/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,akutz/go,nawawi/go,CAFxX/go,josharian/go,nawawi/go,akutz/go,CAFxX/go,josharian/go,akutz/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,golang/go,golang/go,golang/go,josharian/go,golang/go,golang/go,CAFxX/go,nawawi/go,josharian/go,cloudflare/go,akutz/go,nawawi/go,nawawi/go,josharian/go,josharian/go,CAFxX/go,cloudflare/go,akutz/go,CAFxX/go