No explanation of standard TimeOnly formats

### Type of issue

Missing information

### Description

It says

> The accepted standard formats are 'r', 'R', 'o', 'O', 't' and 'T'.

It would be helpful if the documentation said what those formats were

### Page URL

https://learn.microsoft.com/en-us/dotnet/api/system.timeonly.tostring?view=net-8.0#system-timeonly-tostring(system-string)

### Content source URL

https://github.com/dotnet/dotnet-api-docs/blob/main/xml/System/TimeOnly.xml

### Document Version Independent Id

ab57dbcd-ba49-5b66-9f13-f273529a7126

### Article author

@dotnet-bot