Rewrite the dynamic dispatch section to focus on usage rather than implementation.

[Ms2ger]

No description provided.

[rust-highfive]

r? @nikomatsakis

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[Ms2ger]

r? @steveklabnik

<njn> Ms2ger: that's definitely an improvement
<njn> Ms2ger: I suggest changing the method name to something other than `method`, because that becomes unclear in the representation section
<njn> Ms2ger: also, is the representation section important to be able to use trait objects? If not, a "you don't really need to know these details" warning would be helpful
<huon> njn: it is helpful to understand certain errors (particularly those around object safety)
<huon> but that's all, AFAIK

If you have wording that would be good for that, please let me know.

[steveklabnik]

Why remove this?

[steveklabnik]

I like this overall, my only beef is using that syntax before explaining it. What do you think?

[steveklabnik]

r=me after a squash

[steveklabnik]

@bors: r+ 3ac89d3 rollup