Update dm doc to make it clear for is-sharding and shard-mode configuration usage

[fubinzh]

Change Request

This repository is ONLY used to solve issues related to TiDB Data Migration (DM) documentation.
For other issues (related to TiDB, PD, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.
   We need to make it clear how is-shardning and shard-mode configuration items should be used in DM task configuration file:

• is-shardning configuration item is not listed in current DM task configuration file
• for shard-mode, it is stated that The "pessimistic" mode is used by default., this may cause some confusion that user doesn't need to configure it.
  refer to:
  https://docs.pingcap.com/zh/tidb-data-migration/stable/task-configuration-file-full
  https://docs.pingcap.com/zh/tidb-data-migration/stable/task-configuration-guide

2. Describe your suggestion or addition.
   Make it clear how these two config should be used.

3. Provide some reference materials (documents, websites, etc) if you could.