docs: document max entries for `/blockchain` RPC (#5356)

Closes #5339

rpc/openapi/openapi.yaml

@@ -605,7 +605,7 @@ paths:
                 $ref: "#/components/schemas/ErrorResponse"
   /blockchain:
     get:
-      summary: Get block headers for minHeight <= height <= maxHeight.
+      summary: Get block headers (max: 20) for minHeight <= height <= maxHeight.
       operationId: blockchain
       parameters:
         - in: query
@@ -623,7 +623,9 @@ paths:
       tags:
         - Info
       description: |
-        Get Blockchain info.
+        Get block headers for minHeight <= height maxHeight. 
+
+        At most 20 items will be returned.
       responses:
         "200":
           description: Block headers, returned in descending order (highest first).