Add type definition example to the manual, consider changing the term to "type alias".

[nejucomo]

The Type Definitions section of the Manual would benefit from some code examples, especially those that clarify how traits and impls interact with the new name.

I find the term "type definition" confusing because this section seems to be describing what I think of as "type aliases". Aren't enum and struct also examples of "type definitions"? If so, is it worth changing this terminology?

[nejucomo]

Here are some relevant examples thanks to @Kimundi.

[alexcrichton]

cc @steveklabnik

[steveklabnik]

Yeah, I'm not sure what the right terminology is here. Hmm.

[steveklabnik]

@aturon what do you think?

[aturon]

+1 for "type alias" -- much clearer.