dumpling: update dumpling document on how to reduce memory usage (#5309)

[ti-srebot]

cherry-pick #5309 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In docs repo:
git pr https://github.com/pingcap/docs/pull/5380

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/docs.git pr/5380:release-4.0-f5cb16e53074

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What is changed, added or deleted? (Required)

Update dumpling document on how to reduce memory usage. Make sure users can get this important infomation.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from: dumpling: update dumpling document on how to reduce memory usage docs-cn#5860
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-srebot]

@TomShawn you're already a collaborator in bot's repo.

[TomShawn]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

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Commit hash: c74d8fb