[doc] Explain that the opposite of --no-overwrites is --no-continue.

[mk-pmb]

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• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

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• I am the original author of this code and I am willing to release it under Unlicense
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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

Helps users understand --no-continue and thereby maybe solves #32783.