Bug 1599402 - Add localization comments. r=Honza,flod.

Differential Revision: https://phabricator.services.mozilla.com/D54954

--HG--
extra : moz-landing-system : lando

devtools/client/locales/en-US/netmonitor.properties

@@ -330,15 +330,16 @@ netmonitor.timings.requestTiming=Request Timing
 netmonitor.timings.serverTiming=Server Timing
 
 # LOCALIZATION NOTE (netmonitor.timings.queuedAt): This is relative queued time to the
-# first request
+# first request. %S is time expressed in milliseconds or minutes.
 netmonitor.timings.queuedAt=Queued: %S
 
-# LOCALIZATION NOTE (netmonitor.timings.startedAt): Related to first request,
-# when the request actually started
+# LOCALIZATION NOTE (netmonitor.timings.startedAt): Relative to the first request,
+# when the request actually started. %S is time expressed in milliseconds or minutes.
 netmonitor.timings.startedAt=Started: %S
 
-# LOCALIZATION NOTE (netmonitor.timings.downloadedAt): Related to first request,
-# when the request actually finished downloading
+# LOCALIZATION NOTE (netmonitor.timings.downloadedAt): Relative to first request,
+# when the request actually finished downloading.
+# %S is time expressed in milliseconds or minutes.
 netmonitor.timings.downloadedAt=Downloaded: %S
 
 # LOCALIZATION NOTE (networkMenu.millisecond): This is the label displayed