Add documentation for CppInlineNamespaceAttribute

[MSDN-WhiteKnight]

Introduced in: dotnet/runtime#781

[opbld30]

Docs Build status updates of commit ae8c150:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Runtime.CompilerServices/CppInlineNamespaceAttribute.xml	✅Succeeded	View

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[carlossanlop]

@MSDN-WhiteKnight thank you for your PR. It looks good to me.

We are temporarily merging .NET 5.0 changes into the net5-rc2 branch, not master. I talked to @Anipik, who is the main area owner of System.Runtime.CompilerServices, and he agreed to cherry-pick your change in this PR and include it in a bigger PR he is going to submit soon to the net5-rc2 branch, documenting everything new in that namespace.

Again, thank you for your contribution!

[carlossanlop]

PR cherry-picking this change: #4902

Closing this PR now.