For #15369: Add documentation for synced tabs metrics.

app/metrics.yaml

@@ -334,7 +334,7 @@ events:
   synced_tab_opened:
     type: event
     description: >
-      A counter that indicates how many synced tabs a user has opened
+      An event that indicates that a synced tab was opened.
     send_in_pings:
       - metrics
     bugs:

docs/metrics.md

@@ -261,7 +261,7 @@ The following metrics are added to the ping:
 | browser.search.ad_clicks |[labeled_counter](https://mozilla.github.io/glean/book/user/metrics/labeled_counters.html) |Records clicks of adverts on SERP pages. The key format is ‘<provider-name>’.  |[1](https://github.com/mozilla-mobile/fenix/pull/10112), [2](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |2 |
 | browser.search.in_content |[labeled_counter](https://mozilla.github.io/glean/book/user/metrics/labeled_counters.html) |Records the type of interaction a user has on SERP pages.  |[1](https://github.com/mozilla-mobile/fenix/pull/10167), [2](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |2 |
 | browser.search.with_ads |[labeled_counter](https://mozilla.github.io/glean/book/user/metrics/labeled_counters.html) |Records counts of SERP pages with adverts displayed. The key format is ‘<provider-name>’.  |[1](https://github.com/mozilla-mobile/fenix/pull/10112), [2](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |2 |
-| events.synced_tab_opened |[event](https://mozilla.github.io/glean/book/user/metrics/event.html) |A counter that indicates how many synced tabs a user has opened  |[1](https://github.com/mozilla-mobile/fenix/pull/16727)||2021-05-10 |2 |
+| events.synced_tab_opened |[event](https://mozilla.github.io/glean/book/user/metrics/event.html) |An event that indicates that a synced tab was opened.  |[1](https://github.com/mozilla-mobile/fenix/pull/16727)||2021-05-10 |2 |
 | events.total_uri_count |[counter](https://mozilla.github.io/glean/book/user/metrics/counter.html) |A counter of URIs visited by the user in the current session, including page reloads. This does not include background page requests and URIs from embedded pages or private browsing but may be incremented without user interaction by website scripts that programmatically redirect to a new location.  |[1](https://github.com/mozilla-mobile/fenix/pull/1785), [2](https://github.com/mozilla-mobile/fenix/pull/8314), [3](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |2 |
 | metrics.adjust_ad_group |[string](https://mozilla.github.io/glean/book/user/metrics/string.html) |A string containing the Adjust ad group ID from which the user installed Fenix. This will not send on the first session the user runs. If the install is organic, this will be empty.  |[1](https://github.com/mozilla-mobile/fenix/pull/9253), [2](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |2 |
 | metrics.adjust_campaign |[string](https://mozilla.github.io/glean/book/user/metrics/string.html) |A string containing the Adjust campaign ID from which the user installed Fenix. This will not send on the first session the user runs. If the install is organic, this will be empty.  |[1](https://github.com/mozilla-mobile/fenix/pull/5579), [2](https://github.com/mozilla-mobile/fenix/pull/15713#issuecomment-703972068)||2021-08-01 |1 |