Document classes that the user cannot instantiate

[microbit-matt-hillsdon]

Button, the pins classes, the mem class in machine and probably others are all classes we need in the API docs but aren't intended for the user to create. Let's add some clear consistent documentation explaining this.

[microbit-matt-hillsdon]

uname_result is another example here of a class that only exists (in MicroPython at least) to document the structure of a return value. It's even less instantiable than the others.