List of extension methods missing generic specializations

Consider https://docs.microsoft.com/en-us/dotnet/api/system.span-1?view=net-6.0. It has a list of extension methods that apply to `Span`, e.g.

but nowhere do we see extensions on `Span` for a specific `T`. For example, many of those methods are on `MemoryExtensions`, which also has a bunch of methods specific to `Span` and `ReadOnlySpan`, but those methods aren't displayed anywhere. The docs should be updated to include such specializations... could be in the same list of extensions, could be in a separate list per T (e.g. one list for char), etc.

cc: @gewarren