Port System triple slash to docs

[carlossanlop]

Ported using DocsPortingTool.

Found a few APIs that could be ported.

[opbld33]

Docs Build status updates of commit d2b1812:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
xml/System/HashCode.xml	⚠️Warning	View	Details
xml/System/ArgumentNullException.xml	✅Succeeded	View
xml/System/BinaryData.xml	✅Succeeded	View
xml/System/MemoryExtensions.xml	✅Succeeded	View
xml/System/String.xml	✅Succeeded	View

xml/System/HashCode.xml

• Line 0, Column 0: [Warning-ECMA2Yaml_UidAssembly_NotMatched] System.HashCode's moniker dotnet-plat-ext-3.0 can't match any assembly.
• Line 0, Column 0: [Warning-ECMA2Yaml_UidAssembly_NotMatched] System.HashCode's moniker dotnet-plat-ext-3.1 can't match any assembly.
• Line 0, Column 0: [Warning-ECMA2Yaml_UidAssembly_NotMatched] System.HashCode's moniker dotnet-plat-ext-5.0 can't match any assembly.
• Line 0, Column 0: [Warning-ECMA2Yaml_UidAssembly_NotMatched] System.HashCode's moniker dotnet-plat-ext-6.0 can't match any assembly.

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