Exclude quadruple slash //// and /*** from documentation comments

[KalleOlaviNiemitalo]

Describe the bug

It isn't clear from §D.2 (Documentation comments / Introduction) whether a comment that starts with four slashes //// is a documentation comment. The text talks about comments "that start with three slashes" and, well, if there are four slashes, then three of those are at the start. The expository Single_Line_Doc_Comment grammar rule matches this syntax too, as a slash / can be an Input_Character.

Likewise /*** and Delimited_Doc_Comment.

However, Roslyn does not parse e.g. //// <hello> as a documentation comment, and "Microsoft (R) Visual C# 2005 Compiler version 8.00.50727.9149 for Microsoft (R) Windows (R) 2005 Framework version 2.0.50727" doesn't either. The quadruple-slash syntax has been used for commenting out code, but I forget where I saw that recommendation.

Example

//// <not a doc comment>
class C {
    /***  <not a doc comment> */
    int i;
}

Expected behavior

Declare that a Single_Line_Comment that starts with four or more slashes is not a documentation comment. In text and also in the Single_Line_Doc_Comment grammar rule.

Likewise, declare that a Delimited_Comment that starts with a slash and three or more asterisks is not a documentation comment.

Additional context

Roslyn excludes those here:

• //// at https://github.com/dotnet/roslyn/blob/version-2.9.0/src/Compilers/CSharp/Portable/Parser/Lexer.cs#L2238-L2241
• /*** (and /**/) at https://github.com/dotnet/roslyn/blob/version-2.9.0/src/Compilers/CSharp/Portable/Parser/Lexer.cs#L2261-L2264