Update ByteStore documentation.

pkg/analyzer/lib/src/dart/analysis/byte_store.dart

Index: pkg/analyzer/lib/src/dart/analysis/byte_store.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/byte_store.dart b/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
index 698b5666a09911fd705b61dc57c2e9c4a6631ea7..fb07ac06b4459176ac42e212cf658bdb58f62f0f 100644
--- a/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/byte_store.dart
@@ -8,10 +8,13 @@ import 'dart:collection';
  * Store of bytes associated with string keys.
  *
  * Each key must be not longer than 100 characters and consist of only `[a-z]`,
- * `[0-9]`, `.` and `_` characters. It cannot have the sequence `..` in it.
+ * `[0-9]`, `.` and `_` characters. The key cannot be an empty string, the
+ * literal `.`, or contain the sequence `..`.
  *
  * Note that associations are not guaranteed to be persistent. The value
  * associated with a key can change or become `null` at any point in time.
+ *
+ * TODO(scheglov) Research using asynchronous API.
  */
 abstract class ByteStore {
   /**