docs(wildlife): cite the documented iNaturalist UTFGrid endpoint and Acartia's key request

/grid/{z}/{x}/{y}.grid.json is documented in the v1 and v2 API docs as
Grid Tiles UTFGrid; only its per-cell records are unspecified, which is
what the schema guard covers. DATA_SOURCES.md now also notes that
Acartia's README asks API users to create an account and an API key,
although /sightings/current answers without one today.

docs/CURRENT-STATE.md

@@ -160,7 +160,8 @@ adjacent-zoom tiles publish before network waits; each arriving tile replaces
 its covered area, including healthy empty tiles. Grid tiles publish even while
 the independent observation request is pending or failed. Enable waits for one
 camera settle; camera acquisition uses moveEnd with a 600 ms debounce. The
-undocumented UTFGrid endpoint has a schema guard; see `KNOWN-ISSUES.md`.
+documented UTFGrid endpoint's unspecified cell records have a schema guard;
+see `KNOWN-ISSUES.md`.
 
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