docs: Document thread safety of library (#1566)

* Document thread safety of library

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google-cloud-storage/src/main/java/com/google/cloud/storage/package-info.java

@@ -50,6 +50,12 @@
  * ID</a> and <a href="https://github.com/googleapis/google-cloud-java#authentication">provide
  * credentials</a>.
  *
+ * <p>Operations in this library are generally thread safe, except for the use of <a
+ * href="https://github.com/googleapis/java-storage/blob/main/google-cloud-storage/src/main/java/com/google/cloud/storage/BlobReadChannel.java">
+ * BlobReadChannel</a> and <a
+ * href="https://github.com/googleapis/java-storage/blob/main/google-cloud-storage/src/main/java/com/google/cloud/storage/BlobWriteChannel.java">
+ * BlobWriteChannel</a>.
+ *
  * @see <a href="https://cloud.google.com/storage/">Google Cloud Storage</a>
  */
 package com.google.cloud.storage;