Clarify the behavior of overlay-ports.

[BillyONeal]

These clarifications were originally written for microsoft/vcpkg-tool#1534 , which we have decided not to do. I kept things which were useful improvements on their own.

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Learn Build status updates of commit 73caad9:

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vcpkg/commands/common-options.md	✅Succeeded	View
vcpkg/commands/help.md	✅Succeeded	View
vcpkg/concepts/overlay-ports.md	✅Succeeded	View
vcpkg/concepts/package-name-resolution.md	✅Succeeded	View
vcpkg/reference/vcpkg-configuration-json.md	✅Succeeded	View
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PRMerger Results

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[learn-build-service-prod]

Learn Build status updates of commit 6962922:

✅ Validation status: passed

File	Status	Preview URL	Details
vcpkg/commands/common-options.md	✅Succeeded	View
vcpkg/commands/help.md	✅Succeeded	View
vcpkg/concepts/overlay-ports.md	✅Succeeded	View
vcpkg/concepts/package-name-resolution.md	✅Succeeded	View
vcpkg/reference/vcpkg-configuration-json.md	✅Succeeded	View
vcpkg/users/config-environment.md	✅Succeeded	View

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PRMerger Results

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[learn-build-service-prod]

Learn Build status updates of commit 6b6e78b:

✅ Validation status: passed

File	Status	Preview URL	Details
vcpkg/commands/common-options.md	✅Succeeded	View
vcpkg/commands/help.md	✅Succeeded	View
vcpkg/concepts/overlay-ports.md	✅Succeeded	View
vcpkg/concepts/package-name-resolution.md	✅Succeeded	View
vcpkg/reference/vcpkg-configuration-json.md	✅Succeeded	View
vcpkg/users/config-environment.md	✅Succeeded	View

For more details, please refer to the build report.

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PRMerger Results

Issue	Description
File Change Percent	This PR contains file(s) with more than 30% file change.