Add summary for primitives namespace

[Nick-Stanton]

Summary

Provides missing summary for Microsoft.Extensions.Primitives namespace.

Partial fix for dotnet/runtime#43920

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[ghost]

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Issue Details

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Summary

Provides missing summary for Microsoft.Extensions.Primitives namespace.

Partial fix for dotnet/runtime#43920

Author:	Nick-Stanton
Assignees:	Nick-Stanton
Labels:	area-Extensions-Primitives
Milestone:	-

[opbld34]

Docs Build status updates of commit 052c58e:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/ns-Microsoft.Extensions.Primitives.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

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