docs(canton): document heightId block lookups

modules/sdk-coin-canton/README.md

@@ -23,7 +23,45 @@ const sdk = new BitGoAPI();
 sdk.register('canton', Canton.createInstance);
 ```
 
-## Development
+## Block lookup
+
+Canton block identifiers are ledger offsets rather than conventional blockchain
+block hashes. A ledger can be reset, so the offset is only unique together with
+the ledger version.
+
+When looking up a Canton block, use the transfer's `heightId` value from the
+Get Transfer API response as the `{height}` path parameter. Do not use the
+plain numeric `height` value: it represents only the ledger offset and can
+refer to a different ledger after a reset.
+
+```text
+GET /api/v2/canton/public/block/{heightId}
+```
+
+For example, a transfer response containing:
+
+```json
+{
+  "height": 6291143,
+  "heightId": "006291143-6a85688d879741a95e3940b671adb39b"
+}
+```
+
+must be followed by:
+
+```text
+GET /api/v2/canton/public/block/006291143-6a85688d879741a95e3940b671adb39b
+```
+
+A request using `/public/block/6291143` can return `block not found`, even
+when the transfer exists. The `heightId` is the canonical Canton block
+identifier and includes both the offset and the ledger version.
+
+This is a Canton-specific exception to the usual block lookup pattern. For
+other coins, use the block identifier documented for that coin; for coins
+with conventional, non-resetting block heights, the numeric `height` can be
+used. No other coin-specific `heightId` exception is defined by this package.
+
 
 Most of the coin implementations are derived from `@bitgo/sdk-core`, `@bitgo/statics`, and coin specific packages. These implementations are used to interact with the BitGo API and BitGo platform services.