Write up default methods for the tutorial.
Wording tweak suggested by @pcwalton.
tutorial: change `float` to `f32` since float is no longer a type.
tutorial: remove duplicate words.
auto merge of #11270 : lkuper/rust/trait-docs, r=cmr

I've been wanting to explain default methods in the tutorial for ages.  While I was at it I tried to clarify some other parts of the traits section.  I'm sure this could be improved further; suggestions welcome.