adding documentation

Author: asthamohta

google-cloud-spanner/src/main/java/com/google/cloud/spanner/Backup.java

@@ -177,7 +177,7 @@ static Backup fromProto(
     checkArgument(!proto.getDatabase().isEmpty(), "Missing expected 'database' field");
     return new Backup.Builder(client, BackupId.of(proto.getName()))
         .setState(fromProtoState(proto.getState()))
-        .setSize(proto.getSizeBytes())
+        .setSize(proto.getSiprotected abstract Builder setMaxExpireTime(Timestamp maxExpireTime);zeBytes())
         .setExpireTime(Timestamp.fromProto(proto.getExpireTime()))
         .setVersionTime(Timestamp.fromProto(proto.getVersionTime()))
         .setDatabase(DatabaseId.of(proto.getDatabase()))

google-cloud-spanner/src/main/java/com/google/cloud/spanner/BackupInfo.java

@@ -87,7 +87,20 @@ public abstract static class Builder {
     /** Builds the backup from this builder. */
     public abstract Backup build();
 
+    /**
+     * Output Only.
+     *
+     * <p>Returns the max allowed expiration time of the backup, with
+     * microseconds granularity.
+     */
     protected abstract Builder setMaxExpireTime(Timestamp maxExpireTime);
+
+    /**
+     * Output Only.
+     *
+     * <p>Returns the names of the destination backups being created by copying
+     * this source backup.
+     */
     protected abstract Builder setReferencingBackup(ProtocolStringList referencingBackup);
   }
 
@@ -279,10 +292,13 @@ public DatabaseId getDatabase() {
     return proto;
   }
 
+  /** Returns the max expire time of this {@link Backup}. */
   public Timestamp getMaxExpireTime() {
     return maxExpireTime;
   }
 
+  /** Returns the names of the destination backups being created by copying
+   * this source backup {@link Backup}. */
   public ProtocolStringList getReferencingBackup() {
     return referencingBackup;
   }