Duplicate BufferedTask callback docs from moduledocs to callback docs

apps/explorer/lib/explorer/buffered_task.ex

@@ -49,9 +49,44 @@ defmodule Explorer.BufferedTask do
 
   require Logger
 
+  @doc """
+  Populates a task's buffer on boot with an initial set of entries.
+
+  For example, the following callback would buffer all unfetched account balances on startup:
+
+      def init(acc, reducer) do
+        Chain.stream_unfetched_addresses([:hash], acc, fn %{hash: hash}, acc ->
+          reducer.(Hash.to_string(hash), acc)
+        end)
+      end
+
+  The `init/2` operation may be long-running and allows concurrent calls to `Explorer.BufferedTask.buffer/2` for
+  on-demand entries.
+  """
   @callback init(initial :: term, reducer :: function) ::
               {:ok, accumulated_results :: term | initial :: term} | {:error, reason :: term}
 
+  @doc """
+  Invoked as concurrency becomes available with a list of batched entries to be processed.
+
+  For example, the `run/2` callback for above could be written:
+
+      def run(string_hashes, _retries) do
+        case EthereumJSONRPC.fetch_balances_by_hash(string_hashes) do
+          {:ok, results} -> :ok = Chain.update_balances(results)
+          {:error, reason} -> {:retry, reason}
+        end
+      end
+
+  If a task crashes, it will be retried automatically with an increased `retries` count passed in as the second
+  argument. Tasks may also be programmatically retried by returning `{:retry, reason}` from `run/2`.
+
+  ## Returns
+
+   * `:ok` - run was successful
+   * `{:retry, reason}` - run should be retried after it failed due to `reason`
+
+  """
   @callback run(entries :: list, retries :: pos_integer) :: :ok | {:retry, reason :: term} | {:halt, reason :: term}
 
   @doc """