Proposal: <placeholder> documentation element

[sharwell]

Champion spec: #8975

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

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

🔗 placeholder (Sandcastle MAML Guide)
🔗 SA1651: Do not use placeholder elements (StyleCop Analyzers)

[sharwell]

/cc @EWSoftware