[Proposal]: <placeholder> documentation element

* Discussion: https://github.com/dotnet/csharplang/discussions/8974

## Summary
[summary]: #summary

I propose updating the C# language specification to include a mention of a `` element in documentation. This "inline" element identifies content that is not yet finalized and/or needs further review.

One primary advantage of the `` element is documentation generation scenarios. Generated documentation content can be placed inside `` elements to encourage review prior to final acceptance.

🔗 [placeholder (Sandcastle MAML Guide)](http://ewsoftware.github.io/MAMLGuide/html/d995d093-ce18-4a7e-bd03-82c6c0b4fd72.htm)
🔗 [SA1651: Do not use placeholder elements (StyleCop Analyzers)](https://github.com/DotNetAnalyzers/StyleCopAnalyzers/blob/master/documentation/SA1651.md)