Update Utf8String APIs (dotnet#32892)

src/libraries/System.Utf8String.Experimental/README.md

@@ -20,7 +20,7 @@ This can occur if the NuGet client attempts to install a newer version of the pa
 install-package System.Utf8String.Experimental -prerelease -source https://dotnetfeed.blob.core.windows.net/dotnet-core/index.json -version 3.0.0-preview6.19223.2
 ```
 
-Not all of the APIs are hooked up yet, but we have some preliminary APIs that allow experimentation with the feature, including basic creation and inspection of `Utf8String` instances, wrapping a `ReadOnlySpan<byte>` or a `ReadOnlyMemory<byte>` around a `Utf8String` instance, and passing a `Utf8String` instance through `HttpClient`. Full list of APIs available at https://github.com/dotnet/runtime/blob/master/src/libraries/System.Utf8String.Experimental/ref/System.Utf8String.cs.
+Not all of the APIs are hooked up yet, but we have some preliminary APIs that allow experimentation with the feature, including basic creation and inspection of `Utf8String` instances, wrapping a `ReadOnlySpan<byte>` or a `ReadOnlyMemory<byte>` around a `Utf8String` instance, and passing a `Utf8String` instance through `HttpClient`. Full list of APIs available at https://github.com/dotnet/runtime/blob/master/src/libraries/System.Utf8String.Experimental/ref/System.Utf8String.Experimental.cs.
 
 Certain language features also work as expected.
 
@@ -37,4 +37,4 @@ fixed (byte* pUtf8 = s1) { /* use 'pUtf8' here */ }
 GCHandle handle = GCHandle.Alloc(s1, GCHandleType.Pinned);
 ```
 
-For more information on the feature, see https://github.com/dotnet/corefx/issues/30503.
+For more information on the feature and API usage, see https://github.com/dotnet/corefxlab/issues/2350 and https://github.com/dotnet/runtime/issues/933.