There is essentially no language specific documentation in well made APIs. There is a common type system so there is very little that's different. Mainly if you need a semicolon and how you prefer your parentheses.
There are some poorly made APIs, like ASP.NET MVC that exploit C#-specific side effects to compensate for C#'s verbosity. These APIs ignore the common type system and just pass object everywhere, breaking tooling and type safety.
But as long as Xamarin didn't come up with terrible designs, you're all set. And, even if they do rely on something strange, F# is such a better language, that the small setback of dealing with interop (which doesn't require learning C#) is totally worth it.
I'm an F# developer, but still decided to not use F# to build an android app. Dealing with the UI libraries like support.v4 was just too painful from a functional perspective. If you don't need any object oriented dependencies, then it's smooth sailing.
I've only heard that in context of A: missing tooling, B: Microsoft, in their quest to make customers feel C# is still the real language and F#'s just some science toy. I've used F# for a Web UI via WebSharper and it was lovely. It's just a poor reflection on the state of UI kits if they don't work with F#.
There are some poorly made APIs, like ASP.NET MVC that exploit C#-specific side effects to compensate for C#'s verbosity. These APIs ignore the common type system and just pass object everywhere, breaking tooling and type safety.
But as long as Xamarin didn't come up with terrible designs, you're all set. And, even if they do rely on something strange, F# is such a better language, that the small setback of dealing with interop (which doesn't require learning C#) is totally worth it.