Clarify CookieContainer thread-safety

[antonfirsov]

This PR supplements dotnet/docs#45658 by clarifying that CookieContainer methods are thread-safe but also noting the dangers of unanticipaded CookieContainer sharing.

Fixes #1535
Fixes #10770

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/ncl

[learn-build-service-prod]

Learn Build status updates of commit b3f9a00:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
xml/System.Net/CookieContainer.xml	⚠️Warning	View	Details

xml/System.Net/CookieContainer.xml

• Line 0, Column 0: [Warning: disallowed-html-tag - See documentation] HTML tag 'format' isn't allowed. Replace it with approved Markdown or escape the brackets if the content is a placeholder.

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[learn-build-service-prod]

Learn Build status updates of commit 26d016e:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
xml/System.Net/CookieContainer.xml	⚠️Warning	View	Details

xml/System.Net/CookieContainer.xml

• Line 0, Column 0: [Warning: disallowed-html-tag - See documentation] HTML tag 'format' isn't allowed. Replace it with approved Markdown or escape the brackets if the content is a placeholder.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

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[learn-build-service-prod]

Learn Build status updates of commit 5d2c10c:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net/CookieContainer.xml	✅Succeeded	View

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[learn-build-service-prod]

Learn Build status updates of commit d6fac13:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Net/CookieContainer.xml	✅Succeeded	View

For more details, please refer to the build report.

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