Document type fields

[tjhorner]

The fields in the API types don't have very much documentation; it would be ideal for them to inherit their documentation from the official Telegram API docs, perhaps with anything specific to the library as well.

For example, the Message type was documented in #379.

[ilyakaznacheev]

I'll do that.

If you don't mind, I will split the task into a couple of PRs.

[ilyakaznacheev]

@tjhorner done. Added documentation for the types I was able to find in the API.

[tjhorner]

@ilyakaznacheev Awesome, thanks! I will take a look at the PRs sometime next week.

[ilyakaznacheev]

@tjhorner thanks! There is nothing but comments (Godoc), so it probably shouldn't take a huge amount of time.

I've changed some descriptions a bit to make them look more like idiomatic Godoc, rather than just a copy-paste from API docs. But the copying, pasting, and formatting itself took a lot more time, then I expected :)

[tjhorner]

Great work @ilyakaznacheev, thanks again :)