ax(mining): replace prose comment with usage example on isRetryableError

AX principle 2: comments show HOW with real values, not WHAT the
function does — the signature already says that.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/service.go

@@ -125,7 +125,8 @@ func respondWithMiningError(c *gin.Context, err *MiningError) {
 	c.JSON(err.StatusCode(), apiErr)
 }
 
-// isRetryableError determines if an error status code is retryable
+// isRetryableError(http.StatusServiceUnavailable) // => true
+// isRetryableError(http.StatusNotFound)            // => false
 func isRetryableError(status int) bool {
 	return status == http.StatusServiceUnavailable ||
 		status == http.StatusTooManyRequests ||