Update TagName docs to include behavior if tag is not found

[mpdelbuono]

Adds detail to the documentation of TagName.IndexOf to clarify that the method will return -1 if the specified tag was not found.

(No behavior change; this is a documentation change only.)

[mpdelbuono]

@dotnet-policy-service agree company="Microsoft"

[ericstj]

For reference, existing docs are here: https://github.com/dotnet/dotnet-api-docs/blob/46669e697cfde9887769092e3bc42d1fcc475586/xml/System.Diagnostics/TagList.xml#L446 and they do mention a bit more detail, but not the -1 case.

I think we'll need to port the changes made here to the api-docs, unless we enable UseCompilerGeneratedDocXmlFile.

I wonder if there are other IndexOf docs that could use similar detail?

[gewarren]

And yes, until this assembly is marked to use the compiler generated XML file, this change won't automatically be reflected in online docs.

[tarekgh]

/ba-g the iOS failure is not related. CC @matouskozak if want to look.

[tarekgh]

@mpdelbuono thanks for submitting this PR. Is it possible you can help submit a PR to port it to the public docs? basically, you need to change the following file:

https://github.com/dotnet/dotnet-api-docs/blob/46669e697cfde9887769092e3bc42d1fcc475586/xml/System.Diagnostics/TagList.xml#L446