Incorrect signatures for span indexers

[BillWagner]

The signatures for indexers of Span<T> and ReadOnlySpan<T> don't show correctly in API reference.

For Span<T>, it shows as:

public T this[int index] { get; }

When it should be:

public ref T this[int index] { get; }

For ReadOnlySpan<T>, it shows as:

public T& this[int index] { get; }

When it should be:

public ref readonly T this[int index] { get; }

This seems to be caused by incorrect behavior of mdoc, so I have created and issue about this there: https://github.com/mono/api-doc-tools/issues/249. But I thought it might make sense to also track fixing the output here (if and when mdoc is fixed), which is why I created this issue.

Moved from dotnet/docs#5208

/cc @svick

[Powerhelmsman]

@dend Please follow up with this bug.

[svick]

Also, for some types, the return type incorrectly includes namespace.

Specifically, the API reference page for ImmutableList<T>.ItemRef shows its return type as System.Collections.Immutable.T& when it should be ref readonly T.

[dend]

Will work with @joelmartinez on addressing this.

[dend]

Tracked here https://github.com/mono/api-doc-tools/issues/260

[mairaw]

As pointed out by @joelmartinez, issue https://github.com/mono/api-doc-tools/issues/260 was not exactly for this, so I've created a new one to track this particular issue: https://github.com/mono/api-doc-tools/issues/422