x/mobile: Docs imply that methods can take structs

[bemasc]

The gomobile docs say it supports

Any function type all of whose parameters and results have
supported types.

I think this is incorrect: gomobile does not support code like the following

type S struct {
  X int
}

type I interface {
  ReadS(s S)
}

Specifically, it does not appear to support passing a struct by value to a function. (Passing an interface, or a pointer to the struct, is supported.)

Please update the docs to reflect this limitation.

[dmitshur]

/cc @hyangah @eliasnaur