mention implicit Sized bound in more places?

[tlyu]

While looking at #992, I found several other inconsistencies or omissions about implicit Sized bounds. 10.6 (Trait and lifetime bounds) says that ?Sized can be used to remove the implicit Sized bounds on type parameters or associated types. If 10.6 is correct about associated types, there are these omissions elsewhere:

• Chapter 11 (Special types and traits) doesn't mention the implicit Sized bound on associated types, but does mention it for type parameters.
• Neither 6.14 (Generic parameters) nor 6.15 (Associated Items) mentions the implicit Sized bound.
• 10.2 (Dynamically Sized Types) should probably mention the implicit Sized bound for associated types. (It does mention them for type parameters.)

Also, chapter 11 should probably mention the ?Sized syntax for removing the implicit Sized bound.

I'm willing to work on a pull request for these, but it might require coordination with #992. (Alternatively, I could base a pull request off of #992, but I'm not sure how well that would work, especially given the pending requested changes there.)