docs(garmin): describe the service choice, key validity and skipped data

The Garmin page says the key goes only to the service picked, how to
correct a key's validity from the card, that a refused key is retried
daily, that weigh-ins reach back past the first watch, that removed
nights disappear, and where data Garmin does not provide is listed. The
changelog entry names the service choice.

CHANGELOG.md

@@ -40,8 +40,9 @@
   arrive as types of their own (`BODY_BATTERY`, `STRESS_LEVEL`), never mixed
   into the computed scores. Garmin keys expire after 30 to 365 days; the
   card warns a week ahead and asks for a new key when Garmin stops
-  accepting the old one. Accounts on Garmin's China service work too. The
-  key is stored encrypted and only ever sent to Garmin. (#1333)
+  accepting the old one. Accounts on Garmin's China service work too; the
+  key goes only to the service you pick. The key is stored encrypted and
+  only ever sent to Garmin. (#1333)
 - **Training Readiness and acute training load.** The Personal API does
   not carry them, so they are new types (`TRAINING_READINESS`,
   `ACUTE_TRAINING_LOAD`) a Home Assistant bridge can push through an ingest

docs/integrations/garmin.md

@@ -24,9 +24,9 @@ before storing anything, so a mistyped or revoked key is refused on the spot.
 The key is stored encrypted at rest and is never shown again, logged or sent
 anywhere but Garmin.
 
-Accounts registered with Garmin's China service work too: a key the global
-service does not know is tried there once, and the service that accepts it is
-remembered.
+Accounts registered with Garmin's China service work too: pick **Garmin
+Connect China** as the service. The key is only ever sent to the service you
+pick.
 
 After connecting, the last 30 days arrive within a minute and the rest of your
 history follows in the background. The card says "Older history is still
@@ -43,7 +43,10 @@ From seven days before the end the card says when the key expires. When it
 has expired, or you revoked it in Garmin Connect, syncing stops and the card
 asks for a new key: create one, open **Replace key** in the card's menu and
 paste it. Nothing imported is lost. If you did not pick a validity, the card
-says that it cannot warn ahead.
+says that it cannot warn ahead; **Key validity** in the card's menu sets it
+afterwards, together with the day you created the key, without pasting the
+key again. A key Garmin turned down is asked about again once a day, so a
+refusal that passes on its own clears without you doing anything.
 
 ## What comes through
 
@@ -74,6 +77,13 @@ Notes:
 - Days you did not wear the watch import nothing, not zeros.
 - Period days are only added on days you have not logged yourself; your own
   entries are never changed. Cycles Garmin only predicts are not imported.
+- Weigh-ins are read back to the start of Garmin Connect, also from before
+  your first watch, so a scale-only account gets its whole history.
+- A night you delete in Garmin Connect, or that Garmin turns into a nap, is
+  removed here on the next sync.
+- Data Garmin does not provide for your account (a menstrual-cycle summary
+  when you track none, for example) is skipped, does not count as an error,
+  and is listed under **Details for a bug report** on the card.
 - When the same data also arrives through Apple Health or Health Connect, the
   source order in **Settings → Sources** decides which one a chart shows.
   Garmin ranks with the other watches.