Add brief summary and link to 'how to use' for ns-Microsoft.Extensions.ObjectPool.xml

[chrisfcarroll]

Add a one sentence description and a link to how to use it, because using it correctly relies on some guide as to what is correct use

Summary

Describe your changes here.

Fixes #Issue_Number (if available)

[dotnet-issue-labeler]

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.

[chrisfcarroll]

( was there an easy way for me to preview how link urls are processed before i submitted this pr? )

[opbld32]

Docs Build status updates of commit c041920:

❌ Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
xml/ns-Microsoft.Extensions.ObjectPool.xml	❌Error		Details

xml/ns-Microsoft.Extensions.ObjectPool.xml

• Line 0, Column 0: [Error-ECMA2Yaml_InternalError]

Intenal Several Error: System.Xml.XmlException: 'xref' is an undeclared prefix. Line 3, position 15.
   at System.Xml.XmlTextReaderImpl.Throw(Exception e)
   at System.Xml.XmlTextReaderImpl.LookupNamespace(NodeData node)
   at System.Xml.XmlTextReaderImpl.ElementNamespaceLookup()
   at System.Xml.XmlTextReaderImpl.ParseElement()
   at System.Xml.XmlTextReaderImpl.ParseElementContent()
   at System.Xml.Linq.XContainer.ReadContentFrom(XmlReader r)
   at System.Xml.Linq.XDocument.Load(XmlReader reader, LoadOptions options)
   at System.Xml.Linq.XDocument.Parse(String text, LoadOptions options)
   at ECMA2Yaml.ECMALoader.LoadType(FileItem typeFile)
   at ECMA2Yaml.ECMALoader.LoadTypes(String basePath, Namespace ns)

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[opbld33]

Docs Build status updates of commit 04e035c:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/ns-Microsoft.Extensions.ObjectPool.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.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel