Ensure directives are sufficiently documented

[jonsequitur]

Current docs are here and mention directives but not in detail: https://learn.microsoft.com/en-us/dotnet/standard/commandline/use-middleware

[KalleOlaviNiemitalo]

Especially any directives used by dotnet-suggest should be documented so that they can be compatibly implemented in applications that do not use this library (or .NET at all).