Clarify format! implicit positional references

It was a little ambiguous before how explicitl positional parameters and
implicit positional parameters intermingled, and this clarifies how the two
intermingle. This also updates a little bit of documentation/code examples
elsewhere as well.
auto merge of #9644 : alexcrichton/rust/clarify, r=huonw

It was a little ambiguous before how explicitl positional parameters and
implicit positional parameters intermingled, and this clarifies how the two
intermingle. This also updates a little bit of documentation/code examples
elsewhere as well.