MutableMethodReturnType documentation should mention that the warning may not apply to APIs.

Error Prone 2.2.0 can report the warning MutableMethodReturnType on public methods that are part of an API. For example, it can suggest changing a return type from `Collection` to `ImmutableCollection`, where `ImmutableCollection` is a class from Guava. However, the advice in the MutableMethodReturnType [documentation](https://github.com/google/error-prone/blob/d7845cb7f9c82a6e3fef772128d001f5e6fb2633/docs/bugpattern/MutableMethodReturnType.md) may not apply to API methods if the library depends on Guava but avoids exposing Guava types in its API. There are several reasons for a library to not expose Guava, including shading and giving the library the flexibility to change the collection implementation or remove the Guava dependency in the future.