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.

src/libstd/fmt/mod.rs

@@ -56,6 +56,21 @@ example, the format string `{} {} {}` would take three parameters, and they
 would be formatted in the same order as they're given. The format string
 `{2} {1} {0}`, however, would format arguments in reverse order.
 
+Things can get a little tricky once you start intermingling the two types of
+positional specifiers. The "next argument" specifier can be thought of as an
+iterator over the argument. Each time a "next argument" specifier is seen, the
+iterator advances. This leads to behavior like this:
+
+```rust
+format!("{1} {} {0} {}", 1, 2) // => ~"2 1 1 2"
+```
+
+The internal iterator over the argument has not been advanced by the time the
+first `{}` is seen, so it prints the first argument. Then upon reaching the
+second `{}`, the iterator has advanced forward to the second argument.
+Essentially, parameters which explicitly name their argument do not affect
+parameters which do not name an argument in terms of positional specifiers.
+
 A format string is required to use all of its arguments, otherwise it is a
 compile-time error. You may refer to the same argument more than once in the
 format string, although it must always be referred to with the same type.
@@ -71,6 +86,14 @@ argument list and have the syntax:
 identifier '=' expression
 ```
 
+For example, the following `format!` expressions all use named argument:
+
+```rust
+format!("{argument}", argument = "test")       // => ~"test"
+format!("{name} {}", 1, name = 2)              // => ~"2 1"
+format!("{a:s} {c:d} {b:?}", a="a", b=(), c=3) // => ~"a 3 ()"
+```
+
 It is illegal to put positional parameters (those without names) after arguments
 which have names. Like positional parameters, it is illegal to provided named
 parameters that are unused by the format string.