Documentation blocks should be placed on top of the entity they describe

Currently, [documentation blocks](https://github.com/google/emboss/blob/master/doc/language-reference.md#documentation) are placed underneath the entities they describe. This is quite confusing and the opposite of most commenting conventions in various programming languages. I think it makes sense to have a separate symbol for documentation blocks (e.g. `#` for comments and `--` for documentation blocks). However, the position should remain on top of the entity they describe.

I think the one exception here would be to have a documentation block within the top of a `struct`, `enum`, `bits`, etc, kind of like a docstring in python. However, enabling that one exception may make the implementation more difficult so we can default to the top of the entity instead, imo.