docs: Add doc for equals method in blob (#311)

Author: michiboo

google-cloud-storage/src/main/java/com/google/cloud/storage/Blob.java

@@ -916,6 +916,10 @@ public Builder toBuilder() {
     return new Builder(this);
   }
 
+  /**
+   * Returns true if obj instance {@code Blob.toPb()} metadata representation and {@code
+   * Blob.options} instance of StorageOptions are both equal.
+   */
   @Override
   public final boolean equals(Object obj) {
     if (obj == this) {