Add remaining missing docs in System

[buyaa-n]

Summary

Add remaining missing docs in System mentioned in https://github.com/dotnet/runtime/issues/105987

Fixes https://github.com/dotnet/runtime/issues/105987

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Tagging subscribers to this area: @dotnet/area-system-runtime

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Learn Build status updates of commit 267b4e0:

:x: Validation status: errors

Please follow instructions here which may help to resolve issue.

File	Status	Preview URL	Details
[]()	:x:Error		Details
xml/System/MemoryExtensions.xml	:x:Error		Details

• Line 0, Column 0: [Error: ECMA2Yaml_File_LoadFailed] Failed to load 1 files, aborting...

xml/System/MemoryExtensions.xml

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

  Intenal Several Error: System.Xml.XmlException: Name cannot begin with the '<' character, hexadecimal value 0x3C. Line 4967, position 92.
  at System.Xml.XmlTextReaderImpl.Throw(Exception e)
  at System.Xml.XmlTextReaderImpl.Throw(String res, String[] args)
  at System.Xml.XmlTextReaderImpl.ParseQName(Boolean isQName, Int32 startOffset, Int32& colonPos)
  at System.Xml.XmlTextReaderImpl.ThrowTagMismatch(NodeData startTag)
  at System.Xml.XmlTextReaderImpl.ParseEndElement()
  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)

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.

For any questions, please:

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[learn-build-service-prod[bot]]

Learn Build status updates of commit f839563:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System/Delegate.xml	:white_check_mark:Succeeded	View
xml/System/Delegate+InvocationListEnumerator`1.xml	:white_check_mark:Succeeded	View
xml/System/MemoryExtensions.xml	:white_check_mark:Succeeded	View
xml/System/MemoryExtensions+SpanSplitEnumerator`1.xml	:white_check_mark:Succeeded	View
xml/System/Type.xml	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod[bot]]

Learn Build status updates of commit 329a1da:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System/Delegate.xml	:white_check_mark:Succeeded	View
xml/System/Delegate+InvocationListEnumerator`1.xml	:white_check_mark:Succeeded	View
xml/System/MemoryExtensions.xml	:white_check_mark:Succeeded	View
xml/System/MemoryExtensions+SpanSplitEnumerator`1.xml	:white_check_mark:Succeeded	View
xml/System/Type.xml	:white_check_mark:Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel